The "Unknown Days" of the Jewish Calendar
This coming week, an unsuspecting person wishing to catch a minyan, who walks into a random shul in many places around the world, might be in for a surprise. After the Shemoneh Esrei prayer on Sunday there will be no Tachanun. On Monday there will be Selichos; and on Thursday there again won’t be Tachanun! Why would this be? No Tachanun generally signifies that it is a festive day; yet, no other observances are readily noticeable. As for the reciting of Selichos on Monday, they are usually reserved for a fast day; yet no one seems to be fasting! What is going on?
The answer is that three separate specific “Unknown Days” happen to fall out on during this apparently chock-full week.
Sunday, the 14th of Iyar, is dubbed ‘Pesach Sheini’, which commemorates the day when those who were unable to bring the annual Korban Pesach (special Pesach sacrifice - Pascal lamb) at its proper time (Erev Pesach), were given a second chance to offer this unique Korban. Although technically not considered a true holiday, and despite the fact that it is (astonishingly!) not listed in the traditional halachic sources as one of the days that Tachanun is not recited, nevertheless, the custom for many is not to recite Tachanun on Pesach Sheini, in order to ‘tap into’ the merit that this particular day had during the times of the Beis HaMikdash. Although the Pri Megadim maintains that Tachanun should be recited on Pesach Sheini - as the original day it is commemorating was only meant for individuals in unique circumstances, and its absence of mention in the traditional halachic sources as one of the days that Tachanun is not recited speaks volumes, nevertheless, most authorities rule that one should not say Tachanun on this day.
There is an interesting common custom associated with Pesach Sheini - eating Matzah; but that is not noticeable in shul. Another interesting fact about Pesach Sheini is that it is commonly considered the Yahrtzeit of Rabi Meir Baal HaNeis, and many visit his kever in Teveria on that day.
This coming Monday will be the third and final day of a semi-annual period that many do not even realize exists: a series of “Days of Tefilla” colloquially known as BeHa”B. This acronym stands for Monday (Beis - 2nd day of the week), Thursday (Hei - 5th day of the week), and the following Monday (Beis). These days are observed on the first Monday, Thursday, and Monday following the earliest Rosh Chodesh after Pesach (Rosh Chodesh Iyar), and likewise after Sukkos (Rosh Chodesh Marcheshvan).
The custom of utilizing these specific days for prayer and supplication is already mentioned almost 900 years ago by the Baalei Tosafos, its importance reiterated by the Tur, and unbeknownst to many, actually codified in halacha by the Shulchan Aruch as a proper minhag.
Whose Minhag Is It, Anyway?
Although both the Tur and Rema refer to BeHa”B as a German-French custom, and the Shulchan Gavoah (cited l’maaseh by the Kaf Hachaim) writes that this was not the minhag in Spain, implying that Sefardim are not beholden to keep BeHa”B, nevertheless, it must be noted that the Shulchan Aruch himself must have felt strongly about this minhag of BeHa”B, as aside for spending a whole siman on it – Orach Chaim 492, he also refers to it and its importance in at least two other locations – Orach Chaim 429: 2 and 566: 2 - 4. However, in the latter citing he does refer to fasting b’tzibbur for BeHa”B as an ‘Ashkenazic minhag’.
Later Sefardic authorities as well, including the Knesses HaGedolah, Pri Chodosh and Chida (ad loc.) also discuss its importance. Therefore, this author finds it interesting that the general Sefardic minhag is not to recite BeHa”B. In fact, the Siman referring to BeHa”B - Orach Chaim 492, is noticeably absent from the Yalkut Yosef Kitzur Shulchan Aruch, implying that it is not relevant to the average Sefardi.
And it is not just the general Sefardi populace who do not recite BeHa”B. Already in his time, the Taz noted that even among Ashkenazic communities BeHa”B observance was not widespread. More recently, Rav Menashe Klein zt”l, in a side point to the issue being addressed in a teshuva, maintains that one who does not observe BeHa”B is not considered “Poresh Min HaTzibbur” (separating himself from the general community), as even nowadays its observance is not prevalent. But, among many Yeshiva communities, and especially in Eretz Yisrael, reciting BeHa”B twice annually is de rigueur.
The reason most commonly mentioned by the Poskimwhy these days of prayer are following the holidays of Pesach and Sukkos, is that over the extended holidays, when there is a mitzvah of feasting and simcha, it is more likely that people may have stumbled in some area due to improper behavior and inadvertent sin, and BeHa”B is meant to help rectify any possible offense. This idea is based on Iyov (Ch. 1: verse 5) who would bring Korbanos after “Yemei Mishteh”, or “Days of Feasting”. These sins might be due to a variety of probable offenses including: mingling - at a festive meal or even at a Drasha in the Shul (!), Yom Tov and Chol Hamoed related transgressions, Chillul Hashem, or overabundance of merriment and gastronomical pleasures.
Other explanations offered to explain why BeHa”B was established include:
- To entreat Hashem to strengthen our bodies especially at the time of changing seasons when many are likely to get ill.
- To beseech Hashem that we should have abundant harvests after Pesach and plentiful rains after Sukkos.
- To commemorate Queen Esther’s original 3-day fast (which actually was Pesach time, and not Purim time). This is pushed off until the first opportunity after Chodesh Nissan. [The Taanis Esther that we fast before Purim is not really due to Esther’s actual fast as recorded in the Megillah, but rather to commemorate that when the Jews went to war against the anti-Semites of their day, they fasted].
Yet, interestingly, although BeHa”B is traditionally meant to be a fast day, still, very few actually do fast, as in our generations people are considered weaker. Instead, we make do with Selichos (and perhaps Divrei Mussar), with the majority also reciting Avinu Malkeinu. This rationale is found in many sefarim, including the Chavos Yair, Aruch Hashulchan, Mishnah Berurah, and Kaf Hachaim, who cites this as the Minhag of Prague.
This certainly holds true with BeHa”B, as it is based on a minhag and not an actual Biblical source. That is why our fellow walking in would not likely see any other signs of a traditional fast day.
Monday, Thursday, Prayer Days…
Monday and Thursday are considered especially potent days for prayer, as Moshe Rabbeinu climbed Har Sinai to receive the Torah on a Thursday and returned with it on a Monday. A mnemonic to showcase this is the first pasuk read on a public fast day Haftara (Yeshaya Ch. 55: verse 6) “Dirshu Hashem B’H imatzo” - “Seek out Hashem when He is to be found”. The letters Beis and Hei show that an auspicious time when Hashem may be found is on Monday and Thursday; therefore Mondays and Thursdays are preferable for fasting and prayer. For whichever reason BeHa”B was established, we certainly shouldn’t let this golden opportunity for tefilla pass us by. 
The third of these ‘Unknown Days’ with no other obvious observances during davening except the omission of Tachanun on Thursday, is Lag B’Omer [18th of Iyar]. There are several reasons offered for why Tachanun is not said on Lag B’Omer, including:
- It is the day when Rabbi Akiva’s 24,000 students stopped dying.
- It is the day when the Mann (manna) started to fall, feeding Bnei Yisrael in the Midbar.
- It is the day when Rabi Shimon bar Yochai and his son, Rabi Elazar came out of the cave they hid in for 13 years.
- It is Rabi Shimon bar Yochai’s Yahrtzeit;however, it is important to note that this understanding might actually be based on a simple printing mistake, and many authorities, including the Chida and Ben Ish Chai, maintain that Lag B’Omer is not truly hisYahrtzeit.
- It is the day when Rabi Akiva gave Semicha to his five new students (including Rabi Shimon bar Yochai) after the 24,000 died, allowing the Torah’s mesorah to perpetuate.
- It is the day when Rabi Shimon bar Yochai’s hidden Torah, the Zohar, became revealed to the world.
In Eretz Yisrael, this quasi-holiday is widely celebrated, with over half-a million people converging on, to visit, daven, and dance at the grave of Rabi Shimon Bar Yochai, thus lending an electrified atmosphere to the sleepy mountaintop town of Meron in the Galil. Thousands visit the grave of Shimon HaTzaddik in Yerushalayim as well.
Indeed, due to the tremendous influx of Chutznikim to visit Rabi Shimon on ‘his day’, some opine that in Eretz Yisrael, Lag B’Omer has recently become the new ‘third Regel’, far overshadowing the amount of visitors for Shavuos.
With enormous bonfires (mostly built by neighborhood children!) seemingly on almost every street corner, Lag B’Omer in Eretz Yisrael has since also become a pyromaniac’s delight and the firefighters’ and asthmatics’ nightmare.
Other locales have a different sort of custom, such as the one in Krakow, Poland to visit the illustrious Rema’s grave on Lag B’Omer, as it is his Yahrtzeit as well. Yet, in most of Chu”tz La’aretz, there is no other real commemoration or observances of this special day, unless one counts the “traditional” game of baseball and subsequent barbeque. So, it is quite possible that our clueless colleague might just wonder about the omission of Tachanun on Lag B’Omer.
Hopefully, by showcasing these relatively “Unknown Days”, this article should help raise awareness and appreciation of them and their customs. Incredibly, as we are climbing upwards in this unique time period of Sefiras HaOmer, in anticipation of Kabbalas HaTorah, Hashem has given us an opportunity to observe a few special Yemei Tefilla. May we be zocheh to utilize them to their utmost!
L’Iluy Nishmas Rav Shlomo Yoel ben Moshe Dovid zt”l - niftar eruv-Erev Pesach and Yisrael Eliezer ben Zev a"h - my dear Great-Uncle Larry Spitz, who was recently niftar; L’Zechus for Shira Yaffa bas Rochel Miriam v’chol yotzei chalatzeha for a yeshua teikif u’miyad, and l’Refuah Sheleimah for Shoshana Leah bas Dreiza Liba, Rina Geulah bas Dreiza Liba, Rochel Miriam bas Dreiza Liba, Mordechai ben Sarah, and Shayna bas Fayga.
Rabbi Yehuda Spitz, author of M’Shulchan Yehuda on Inyanei Halacha, serves as the Sho’el U’Meishiv and Rosh Chabura of the Ohr Lagolah Halacha Kollel at Yeshivas Ohr Somayach in Yerushalayim. He also writes the ‘Insights Into Halacha’ column for Ohr Somayach’s website: https://ohr.edu/this_week/insights_into_halacha/.
For questions, comments, or for the full mareh mekomos, please contact the author at email@example.com.
 For an explanation of the significance of reciting Tachanun, see recent article titled “Tachanunin Yerushalayim”.
 See Sefer HaTodaah (vol. 2, pg. 253) who explains that it is referred to as ‘Pesach Sheini’ as it is the Pesach that falls out in the second month of the year - Iyar. In the Mishnah and Gemara (ex. Rosh Hashana 18a) it is referred to as “Pesach Kattan” or “Pischa Ze’ira” (the Small Pesach). It is listed in Megillas Taanis (Ch. 2; also cited in Gemara Chullin 129b) that it is a day that hespedim are forbidden (a minor holiday).
 Bamidbar (Parashas Behaalosecha Ch. 9: verses 1 – 15). See Rashi (ad loc. verse 10) and Sefer Hachinuch (Mitzva 380).
 The Aruch Hashulchan (O.C. 131: 12) writes that he is astounded that Pesach Sheini is not listed by any of the traditional Halachic sources - [including the Gemara, Rambam, Tur, Shulchan Aruch, or even the Mishnah Berurah] as one of the days that Tachanun is not said. However, see Shu”t Mor V’Ohalos (Mahadura Kama 39, Ohel Brachos V’Hoda’os, pg. 37 s.v. od) who asserts that there was no reason to list Pesach Sheini as a day with no Tachanun recited, as it is mentioned explicitly in the Torah (meaning it is obvious that there is no Tachanun). Conversely, see Shu”t M’Ma’ayanei Yeshua (3) who posits (and explains at length) that this machlokes (between the Pri Chodosh and Pri Megadim et al.; see next footnote at length) is based on whether or not a “Chag l’Yechidim” would be enough to exempt the tzibbur from Tachanun. He also discusses the differences between Pesach Sheini and Shavuos in regard to reciting Tachanun the week after. See also the Chida’s Moreh B’Etzba (Ch. 8: 222) who states that on Pesach Sheini one should be “yarbeh simcha ketzas, ki kadosh hayom”.
 These Poskim include the Tikkun Yissachar (pg. 32a; referring to Pesach Sheini as somewhat of a ‘Moed’), the Knesses Hagedolah (O.C. 131, Haghos on Beis Yosef 12, citing ‘Minhag Salonika’; however, see Magen Gibborim ad loc. 18 who states that the minhag of Eretz Yisrael, Mitzrayim and Salonika is to recite Tachanun on Pesach Sheini), Pri Chodosh (ad loc. 7), Elyah Rabba (ad loc. 14), Yaavetz (in his Siddur Beis Yaakov vol. 2, Shaar Hayesod, Seder Chodesh Iyar pg. 103: 21), Pardes Eliezer (Pesach Sheini; citing Rav Chaim Vital quoting the Arizal), Shalmei Tzibur (ad loc. 10), Shaarei Teshuva (ad loc. 19), Beis Dovid (Shu”t 53; cited by the Shaarei Teshuva ibid. and Kaf Hachaim ad loc. 99; who explains that there is no real machlokes regarding minhag Salonika, Mitzrayim and Eretz Yisrael, but rather that all agree that Tachanun is not said on the 14th of Iyar as it is the true Pesach Sheini, and those who maintained that it was not recited, were actually referring to the 15th of Iyar), Chida (Machazik Bracha, ad loc. 5), Butchatcher Rav (Eshel Avraham ad loc. s.v. nohagin), Maharsham (Daas Torah ad loc. 7), Shaarei Ephraim (Ch. 10: 27), Shulchan HaTahor (ad loc. 8), the Shoel U’Meishiv (Yosef Daas V’Yad Shaul Y”D 401), Birchas HaPesach (Ch. 16: 5), Aruch Hashulchan (ad loc. 12), Kaf Hachaim (ad loc. 99), Sefer Ishei Yisrael (Ch. 43: 11), Rav Yosef Eliyahu Henkin zt”l’s Ezras Torah Luach (5776, pg. 107; ‘yesh nohagin shelo lomar bo Tachanun’), and the Belz D’var Yom B’Yomo Luach (5776, 14th Iyar). This is also the “Minhag Yerushalayim” - see Shla”h (Maseches Yoma, Perek Derech Chaim, Tochachas Mussar 203), the Tukachinsky Luach Eretz Yisrael (Pesach Sheini), Halichos Shlomo (Tefilla, Ch. 11: 9 and Moadim vol. 2, Ch. 11: 20), and Halichos Even Yisrael (Moadim vol. 1, pg. 218: 1). On the other hand, it is known that the Chazon Ish and Steipler Gaon did recite Tachanun on Pesach Sheini [see sefer Bein Pesach L’Shavuos (in the introduction to the sefer titled ‘Hanhagos HaChazon Ish’ by Rav Chaim Kanievsky), also found at the end of Sefer Imrei Yosher (25), and cited in Orchos Rabbeinu (vol. 1, pg. 68 and new print 5775: vol. 2, pg. 115: 25) and Moadei HaGra”ch (vol. 1: 170, pg. 79)]. Other Gedolim who are quoted as reciting Tachanun on Pesach Sheini include the Chasam Sofer (Minhagim V’Halichos shel Maran HaChasam Sofer (Ch. 11: 3) and the Brisker Rav (cited in Shu”t Pe’as Sadecha vol. 1: 43). There is a recent small sefer titled ‘Pischa Ze’ira’ which devotes considerable attention (pg. 32 – 42 and pg. 90 - 93) to the debate of whether or not to recite Tachanun on Pesach Sheini, and specifically in Yerushalayim, and the various rationales thereof. See also Shu”t Rivevos Efraim (vol. 1: 339 and vol. 258) who explains that Tachanun is recited at the Mincha before Pesach Sheini, even according to the majority opinion that it is not said on Pesach Sheini itself, as opposed to most other Moadim. [Most poskim agree with this as well - see Luach Eretz Yisroel (Chodesh Iyar), Halichos Shlomo (Moadim vol. 2, Ch. 11, Dvar Halacha 29), and Ishei Yisrael (Ch. 25: 18).] Although not widely followed, there is also an opinion based on the Zohar (Parashas Behaalosecha, Rayuh Mehemna, pg. 152b) that Tachanun should not be recited for a full seven days starting from Pesach Sheini [cited in the Shu”t Mor V’Ohalos (ibid.), quoting Rav Mordechai M’Chernobyl; and in Shu”t Divrei Yatziv (O.C. vol. 1: 75, 4 s.v. u’l’zeh), quoting the Divrei Chaim of Sanz. The Pischa Zuta (Birchas HaPesach Ch. 16, footnote 6) cites a different reason as well, quoting the Maharshak in his Imrei Shefer on Parashas Behaalosecha. He adds that the Likutei Mahariach (quoting the sefer HaYashar V’HaTov) was also a proponent of this minhag].
 Eshel Avraham (ibid.).
 Pri Megadim (O.C. 131, M.Z. s.v. mah).
 Such a minhag is commonly traced to a comment by Rav Yaakov Emden in his Siddur Beis Yaakov (Seder Chodesh Iyar 21), opining that Pesach Sheini marked the day when the provisions Bnei Yisrael brought out of Mitzrayim were used up, thus marking the continuation of the miracles of Yetzias Mitzrayim, and of eating Matzah. There is also some debate amongst several Acharonim whether the proper time to fulfill this Matzah Minhag is the day of the 14th of Iyar, as is commonly accepted, or the following evening, the night of the 15th , marking a parallel to the Pesach Seder a month prior. For more on the topic of this minhag, see sefer Zichron Yehuda (pg. 38), Shaar Yissachar (Maamrei Chodesh Iyar, Maamar Pischa Ze’ira 12), Darkei Chaim V’Shalom (Minhagei Yemei Ha’Omer, 631), Lekutei Mahariach (vol. 3, Minhagei Yemei Ha’Omer, Pesach Sheini pg. 44b; also citing the Maharam A”sh), Kli Chemda (Parashas Va’eschanan pg. 26; also citing the Avnei Nezer), Birchas HaPesach (Ch. 16, 5 s.v. yesh and footnote 7), Sefer HaTodaah (vol. 2, pg. 253), Sefer Mataamim (pg. 43), Shu”t Divrei Yisrael (vol. 1: 130; citing precedent from the Ikrei HaDat, O.C. 18: 62, quoting the Pachad Yitzchak that eating Matzah after Pesach is not a violation of ‘Bal Tosif’), Shu”t Rivevos Efraim (vol. 2: 129, 39 & vol. 3: 331), Orchos Rabbeinu (vol. 2, pg. 97, 17), Nitei Gavriel (Pesach vol. 3, pg. 330), Shu”t Divrei Shalom (vol. 4: 87), Shu”t Kinyan Torah B’Halacha (vol. 7: 42, 3), Halichos Even Yisrael (Moadim vol. 1, pg. 218: 3, and footnote 2; who questions how anyone can consider it ‘Bal Tosif’ any more than giving Zeicher L’Machatzis Hashekel on Taanis Esther; however, see sefer Pischa Zeira pg. 187 for a possible distinction between the two), and the Belz D’var Yom B’Yomo Luach (5776, 14th Iyar). See also Moadei HaGra”ch (vol. 1: 164 - 169, pg. 77 - 79), as well as the maamar in Kovetz Moriah (vol. 397 - 399, Nissan 5775) written by its author, Rabbi Matisyahu Gabai, who explains why the Steipler Gaon would eat Matzah on Pesach Sheini, while his brother-in-law, the Chazon Ish, would not. It is also known that the Vilna Gaon (see Maaseh Rav 185) would not eat Matzah after Pesach, to show that he only ate Matzah on Pesach exclusively because it was Hashem’s command. Whatever minhag one follows, all agree that one may not make a bracha of ‘al achilas matzah’ on Pesach Sheini, and if one does, he has made a bracha l’vatallah. See Shu”t Divrei Malkiel (vol. 5: 104) and Shu”t Tzitz Eliezer (vol. 10: 27).
 Although this is widely assumed and cited by many, nevertheless, see Shu”t Yaskil Avdi (vol. 5: 54, 1) citing the Sdei Chemed (vol. 1, Asifas Dinim, Maareches Eretz Yisrael pg. 4a) that the reason masses visit Rabi Meir Baal HaNeis’s kever on Pesach Sheini is that the Beis Knesses and Beis Midrash built over his kever in Teverya were inaugurated on Pesach Sheini and many were kavua a minhag to celebrate every year on that day at his kever, similar to Rabi Shimon bar Yochai’s kever on Lag B’Omer. The Sdei Chemed cites several parallels including celebrating their hidden Torah being revealed to the world (‘ohr ha’meir l’olam, u’meir eini chachamim b’halacha’). Additionally, like Rav Shimon, Rav Meir was one of the five new students of Rabi Akiva who received semicha at that time. It is interesting to note that they do not mention Lag B’Omer being the actual Yahrtzeit of Rabi Shimon bar Yochai (see footnote 31), nor Pesach Sheini the Yahrtzeit of Rabi Meir Baal HaNeis. In fact, the Ben Ish Chai (Year 1, Parashas Vayeishev 28) mentions the minhag to light a candle ‘l’ilui nishmas’ Rabi Meir Baal HaNeis on Rosh Chodesh Teves, implying that that is his true Yahrtzeit. On an interesting side point, there are some who posit [see Rabbi Reuvein Margoliyus’s ‘Lechaker Shemos V’Kinuyim B’Talmud’ (pg. 25: 30, Erech R’ Meir Baal HaNeis) at length] that Rabi Meir Baal HaNeis is not actually the Tanna Rabi Meir, as we do not find that he was called that name in the Gemara, and it was not until somewhat recently when it was assumed that they are one and the same. In fact, there are two different graves found in Eretz Yisrael, one listed as Rabi Meir (in Teverya; actually two - in separate buildings near each other) and another as Rabi Meir Baal HaNeis (in Gush Chalav). Rabbi Margoliyus further (and fascinatingly) opines that the famous Tzedaka in the name of Rabi Meir Baal HaNeis is actually based on an erroneous assumption of seeing the “Kupas Ramba”n” established by the great Rishon, the Ramban, when he moved to Eretz Yisrael following his victory in public debate with the apostate Jew-turned-Franciscan-monk Pablo Christiani. In fact, the Ramban himself wrote (see Toldos Ramban Ch. 4: 13) that he established a yeshiva and a fund to elicit support for it from Jews in Chutz La’aretz. Accordingly, the initials Ramba”n, actually referring to Ra bbi M oshe B en N achman, were mistakenly attributed to Ra bi M eir Baa l HaN eis.
 Although this is the prevalent custom [see Shulchan Aruch and Rema (O.C. 492: 1), Darchei Moshe (O.C. 429: 4), Magen Avraham (O.C. 492: 3), Taz (ad loc. 1), Shach (Y”D 220: end 31), and Mishnah Berurah (ad loc. 3)], on the other hand, there are several communities of Germanic origin who follow the minhag of the Mahari Weil (Shu”t,Dinin V’Halachos 14; cited by the Taz ibid.), and Maharil (Hilchos Shabbos Bereishis 2) that the BeHa”B after Sukkos should be held at the end of Marcheshvan (the last Monday, Thursday, and Monday of the month) only starting after the 17th of the month, instead of at the beginning, as the Mishnah (Taanis 10a) states that 17 Marcheshvan is the starting point for potential BeHa”B fasting during a dearth of rainfall. See the Wurzburger Rav’s Likutei Halevi (pg. 42 and footnote 155 ad loc.) as well as Minhagei Yeshurun (Ch. 16, pg. 29). However, the other poskim (cited above) point out and assert that rain-related fasting should not be dependent upon nor related to the established BeHa”B fasting for post Pesach and Sukkos.
 Tosafos (Kedushin 81a s.v. sakva), Tur and Shulchan Aruch (O.C. 492: 1). Actually, a BeHa”B of sorts is mentioned even further back, in Maseches Sofrim (Ch. 21: 1- 3), but that seems to be for one specific reason: to commemorate Mordechai and Esther’s three-day fast. See footnote 20. There is an interesting machlokes Rishonim between the Rosh (Taanis Ch. 1: 20) and Mordechai (Taanis 629) whether BeHa”B is considered a Taanis Yachid or Taanis Tzibbur.
 Shulchan Gavoah (O.C. 492: 1, cited in Kaf Hachaim 492: 8). This is additionally mentioned in the Sefer HaTodaah (vol. 1, pg 157) and Kitzur Shulchan Aruch of Rav Rafael Baruch Toledano (vol. 4: 330).
 In fact, in Rav Yaakov Hillel’s Ahavat Shalom Luach (5776, pg. 163 s.v. Taanis Sheini Kamma) it simply states: ‘U’Bizmaneinu Bnei Sfard lo nahagu l’hisanos’.
 Taz (O.C. 566: 3), Shu”t Mishnah Halachos (vol. 3: 15, s.v. u’lfa”d). See also Shu”t Igros Moshe (O.C. vol. 1: 106) for an interesting discussion why the Tefilla “Mevorchin BeHa”B” is still recited the Shabbos prior to BeHa”B even when a chosson is present.
 See Rosh (Taanis Ch. 1: 20), Mordechai (Taanis 629), Tur (O.C. 492), Rema (ad loc), Magen Avraham (ad loc, 1), Shulchan Aruch Harav (ad loc, 1), Mishnah Berur ah (ad loc, 1) and Shaarei Efraim (Ch. 10: 46).
 Mingling at a festive meal or even at a Drasha in the Shul is cited by Rashi and Tosafos (Kiddushin 81a s.v. sakva); Yom Tov and Chol Hamoed related transgressions by the Elyah Rabba (O.C. 492: 3; citing the Maaglei Tzedek); Chilul Hashem by the Tur (O.C. 429: 2); overabundance of merriment by the Maharil (Dinei Hayamim Bein Pesach L’Shavuos); and gastronomical pleasures by the Kol Bo.
 See Levush (O.C. 492: 1).
 See Raavyah (Taanis 863), Mordechai (Taanis 629), Shach (Y”D 220: 31 s.v. v’nirah li), and Sefer HaTodaah (vol. 1, pg. 156).
 Maseches Sofrim (Ch. 21: 1 - 3). See also Sefer Minhagei Yisrael (vol. 1, Ch. 26) who opines that according to this reasoning, the reason for the BeHa”B after Sukkos is not for the same reason as the BeHa”B after Pesach. The Taanis Esther that we fast before Purim is not really due to Esther’s actual fast as recorded in the Megillah, but rather to commemorate that when the Jews went to war against the anti-Semites of their day, they fasted [see Mishnah Berurah (686: 2); quoting the Rambam (Hilchos Taaniyos Ch. 5: 5)].
 Shu”t Chavos Yair (126), Aruch Hashulchan (O.C. 492: 2), Mishnah Berurah (566: 6), Kaf Hachaim (O.C. 566: 37; citing the Elyah Rabba, quoting the minhag in Prague), Shu”t Mishnah Halachos (vol. 9: 174), and Nitei Gavriel (Pesach vol. 3, pg. 242). In a similar vein, see also Shu”t Noda B’Yehuda (Kamma O.C. 35, s.v. hinei), Shulchan Aruch Harav (Iggeres HaTeshuva Ch. 3; based on the Yerushalmi Nedarim Ch. 8: 1), and Orchos Rabbeinu (vol. 2, pg. 160, Hilchos Taanis 2). See also Rav Yosef Eliyahu Henkin zt”l’s Ezras Torah Luach (5776; pg. 57 and 106), who adds that since the vast majority of people are not actually fasting, one should replace the word ‘Taanis’ in the Selichos with ‘Tefilla’ instead. This is in accordance with the ruling of the Aruch Hashulchan (O.C. 581: 4) who advises doing the same regarding Selichos in Elul, that it is imperative not to say ‘shekarim’ in these Tefillos. The Ba’er Heitiv (O.C. 492: 3) and Mishnah Berurah (ad loc. 5) imply similarly. The Belz D’var Yom B’Yomo Luach (5776, Iyar, Be”HaB) adds that the Minhag of the Belzer Rebbes is to at least not serve meat on these days.
 See Midrash Tanchuma (Parashas Vayera 16), Tosafos (Bava Kamma 82b s.v. kday shelo), and Mishnah Berurah (134: 6).
 Matteh Moshe (748). See also Shulchan Aruch (O.C. 580: 3, based on Tosefta (Taanis Ch. 2: 5) and Tur, Shulchan Aruch and Rema (O.C. 134: 1) that Mondays and Thursdays are preferable for fasting and prayer. There is even a minhag of fasting every Monday and Thursday for the ‘Churban HaBayis, on the Torah that was burnt, and for Chilul Hashem’. See also Kaf Hachaim (O.C. 580: 32), who concludes this topic stating that the ikar is that there should be Torah along with Teshuvah, and to be ‘laasok tamid b’Toras Chaim’.
 The Shaarei Teshuva (492: 1) cites the Raavad (Tamim Dayim 177) who maintains that “Yechidim and Anshei Maaseh” should keep a BeHa”B after Shavuos as well; yet concludes that the prevailing custom is not to. See also Raavyah (Taanis 863), Mordechai (Taanis 629), Beis Yosef (end O.C. 429), Levush (ad loc. 1), Chok Yaakov (492: 1), Magen Avraham (ad loc. 1), Shulchan Aruch Harav (O.C. 492: 1), Pri Megadim (E.A. ad loc. 1), Chida (Birkei Yosef ad loc. 1), Kaf Hachaim (ad loc. 1) and Mishnah Berurah (ad loc. 1), who all rule that there is no BeHa”B after Shavuos. Apparently, Zeman Mattan Torah affords less opportunity for nichshal.
 So, what happens if one of the BeHa”B days falls out on Pesach Sheini? In such a case, according to the Tukachinsky Luach Eretz Yisrael (and in his Sefer Eretz Yisrael Ch. 18: 1), Rav Shlomo Zalman Auerbach (Halichos Shlomo ibid.) and Rav Yisrael Yaakov Fischer (Halichos Even Yisrael, Moadim vol. 1, pg. 219: 5), aside for Tachanun not being recited, we would also omit the prerequisite Selichos and Avinu Malkeinu. The Shoel U’Meishiv (Hagahos Yad Shaul ibid.) writes similarly. This would certainly hold true according to the opinion of the Cheshek Shlomo (in a letter printed in Sdei Chemed vol. 9, Divrei Chachamim 57) who writes that it is assur to fast on Pesach Sheini, as it is a holiday listed in Megillas Taanis [which he asserts was not battel (see Gemara Rosh Hashana 18b – 19b; which concludes that Megillas Taanis was battel with the destruction of the Beis Hamikdash except for Chanuka and Purim) as it was not established due to miracles; see also Toras Rafael (99)]. See also Shu”t Maharsham (vol. 6: 32), who although arguing that this does not include fasting [meaning Megillas Taanis only addressed hespedim, and did not actually prohibit fasting on Pesach Sheini] nonetheless in an amendment (ad loc.) he writes that he heard that in the Pinkas of the Churva Shul in Yerushalayim it is stated that the minhag in such a case is to only observe two of the three days of BeHa”B. On the other hand, the Butchatcher Rav writes that the minhag in his locale is to make a peshara (compromise): (abridged) Selichos should be recited, but not Tachanun, similar to when a Bris occurs on a fast day, and one may fast if he wishes.
 See Tur, Shulchan Aruch and Rema and relevant commentaries to Orach Chaim (131: 6, and 493: 2).
There are many sefarim available who give full treatment to the topic of Lag B’Omer, including Bein Pesach L’Shavuos and Nitei Gavriel (Pesach vol. 3). It is also worthwhile to read Rabbi Eliezer Brodt’s excellent articles on topic: http://seforim.blogspot.co.il/2011/05/printing-mistake-and-mysterious-origins.html and http://seforim.blogspot.co.il/2008/05/lag-ba-omer-and-upsherins-in-recent.html, and featured in Ami Magazine # 22, as well as the introduction and first chapter to Rabbi Mordechai Zev Trenk’s recent and fascinating Treasures: Illuminating Insights on Esoteric Torah Topics.
 See Meiri (Yevamos 62b), Sefer HaManhig (citing the Baal HaMaor), Biur HaGr”a (O.C. 131: 6 s.v. b’Lag), Pri Chodosh (O.C. 493), Pri Megadim (ad loc. M.Z. 1), Shla”h (Pesachim 525), Kitzur Shulchan Aruch (120: 6), and Mishnah Berurah (493: 8).
 See Shu”t Chasam Sofer (Y”D 233 s.v. amnam yadaati) and Toras Moshe (Parashas Bechukosai, Drush for Chevra Kadisha, pg. 62b).
 See Aruch Hashulchan (O.C. 493: 7).
 See Chayei Adam (vol. 2: 131, 11), Birkei Yosef (O.C. 493: 4), Aruch Hashulchan (ibid.), Kaf Hachaim (ad loc. 27), and Sefer HaTodaah (vol. 2, pg. 256 and 257). The Sefer HaTodaah cites that Rav Ovadia M’Bartenura wrote a letter to his brother in1488 stating that the 18th of Iyar is a Yahrtzeit and was celebrated by masses coming and lighting giant torches. However, it seems unclear whether he was referring to Rabi Shimon bar Yochai or rather Hillel and Shammai, as there was an old minhag to visit their Kevarim near Meron in Iyar around the time of Pesach Sheini and hold large celebrations there, as attested to in a famous travelogue from an anonymous Talmid of the Ramban written in the late 1200s (printed at the end of the recent Beis Midrash L’halacha B’hisyashvus 5764 edition of Kaftor Vaferach; vol. 2; pg. 450, ‘Inyanei Eretz Yisrael’). Interestingly, this is the only celebration that he denotes taking place in Meron that time of year. Curiously, in the very next sentence he briefly mentions that Rabi Shimon bar Yochai and his son, Rabi Elazar are also buried in Meron, with no mention of any annual festivities.
 See the Chida’s Shu”t Maris Ha’ayin (Likutim 7: 8) and Shiyurei Bracha (O.C. 493: 1, Din 2), where he recanted his opinion (Birkei Yosef ibid.) about Lag B’Omer being Rashbi’s Yahrtzeit, and instead maintaining that the celebrations are for different reasons. This is also cited in the Ben Ish Chai’s Shu”t Rav Pe’alim (vol. 1: O.C. beg. 11 s.v. Teshuva), and Rav Yaakov Hillel’s ‘Eid HaGal HaZeh’ at length. They maintain that the first mention of Lag B’Omer being Rabi Shimon bar Yochai’s Yahrtzeit is essentially a printing mistake, and only found in a certain version of Rav Chaim Vital’s Pri Eitz Chaim and not in his authoritative Shaar HaKavannos, which actually lists other reasons for the celebrations. Accordingly, the text of Rav Chaim Vital’s was truly ‘Yom Sameach Rabi Shimon Bar Yochai’, and not ‘Yom Shemeis Rabi Shimon Bar Yochai’. See also Orchos Rabbeinu (new edition 5775; vol. 2 pg. 114 - 115: 19 and 22) for more on why Lag B’Omer is a festive day.
 See Yalkut Me’am Loez (Parashas Emor) and Sdei Chemed (Maareches Eretz Yisrael 6); the Pri Chodosh (ibid.) and Chida (Tuv Ayin 17: 493 and Shiyurei Bracha ibid.) write that it is the day when Rabi Akiva started teaching his five new talmidim.
 See Zohar (Parashas Haazinu, Adra Zutra 241), that on the day of Rav Shimon’s petirah, he revealed his hidden Torah which was ‘nismalei ohr gadol shel simcha’. Additionally, the sun did not set until Rav Shimon finished, after which he passed away. See also Bnei Yissaschar (vol. 1, Maamarei Chodesh Iyar, Maamar 3: 2, 3, 6 & 7) who explains that this is the reason why the minhag is to “light neiros u’me’oros' on this holy day”. As mentioned in footnote 30, Rav Ovadia M’Bartenura attested to this minhag being followed already in his time. The Bnei Yissaschar elaborates further on this minhag, citing several other reasons. Additionally, he posits that Lag B’Omer is also the Rashb”i’s birthday. See also Orchos Rabbeinu (new edition 5775; vol. 2 pg. 114: 19). On the other hand, and as an interesting counter point, albeit not actually discussing Lag B’Omer celebrations, Rav Shmuel Halevi Wosner (Shu”t Shevet Halevi vol. 7: 136) writes that it is not proper to light bonfires and sing and dance around them, as there is a precedentin the Tosefta (Shabbos Ch. 7: 1) to prohibit similar actions, as a potential violation of ‘Darkei Ha’Emori’. See also Rabbi Reuven Chaim Klein’s recent God versus gods: Judaism in the Age of Idolatry (Molech; pg. 337, footnote 445).
 As per the custom mentioned by Rav Chaim Vital in Shaar HaKavannos (pg. 87a; however, he interestingly concludes that it is unclear to him whether or not the Arizal had attained his mastery of chochma and Kabbala when he made this historic pilgrimage); see Ateres Zekainim (O.C. 493: 1), Aruch Hashulchan (O.C. 493: 7), and Kaf Hachaim (O.C. 493: 26). However, it must be noted that there were several authorities who were skeptical at best, if not downright opposed to observing this custom, including the Chasam Sofer (Shu”t Y”D 233 s.v. amnam yadaati and Toras Moshe, Parashas Emor, Hesped for the tragic Earthquake in Tzfas, s.v. ach; he actually maintains that the masses going to Meron on Lag B’Omer were part of the cause of this cataclysm) and the Shoel U’Meishiv (Shu”t vol. 5: 39). [See also Shu”t Sheim Aryeh (O.C. 14) who attempts to answer their criticism.] See also Rav Ovadia Yosef zt”l’s Shu”t Yabea Omer (vol. 5: O.C. 35) and Shu”t Yechaveh Daas (vol. 5: 35), where he wrote very strongly against going up to Meron on Lag B’Omer. It is also well known that many Rabbanim, including Rav Yosef Shalom Elyashiv zt”l, when asked whether one should go to Meron on Lag B’Omer, replied “Rabi Shimon is duh”, pointing to a Gemara (see, for example, Ashrei HaIsh O.C. vol. 3, pg. 432: 17). In other words, if one wants to celebrate Rabi Shimon bar Yochai’s greatness properly on “his day”, they feel that it is preferable to sit and learn his Torah (i.e. learn Gemara). His son-in-law, Rav Chaim Kanievsky, echoed the same sentiments (see Teshuvos HaGra”ch vol. 2, pg. 744 and Moadei HaGra”ch vol. 1: 173, pg. 81). In fact, the Chida (Moreh B’Etzba Ch. 8: 223) mentions such a minhag as well. For a discussion as to the level of Rabi Shimon bar Yochai’s greatness, see Rabbeinu Chananel’s commentary to Sukkah (45b) and Shu”t Rav Pe’alim (vol. 1, Y”D 56 s.v. v’hu pg. 72a) on why he specifically was the one to be zocheh to reveal the Zohar.
 See sefer Bein Pesach L’Shavuos (pg. 302).
Disclaimer: This is not a comprehensive guide, rather a brief summary to raise awareness of the issues. In any real case one should ask a competent Halachic authority.
L'iluy Nishmas the Rosh HaYeshiva - Rav Chonoh Menachem Mendel ben R' Yechezkel Shraga, Rav Yaakov Yeshaya ben R' Boruch Yehuda.