Bar Mitzvah-ed on a Different Day?
As detailed at length in recent articles, our current year, 5781, is full of rare calendarical phenomena. This article sets out to describe an interesting one that can affect Bar Mitzvah bachurim - one that will ensure that they will observe their Bar Mitzvah this year not on their actual birthday, but a day or possibly even a month later. Other years, this divergence may cause Bar Mitzvah observance to occur a day before their actual birthday. Can this truly be? Indubitably so. Read on…
30 Days Has November…
As is well known from Chazal, and actually millennia later corroborated by NASA, the Lunar Month is 29 days, 12 hours and 793 chalakim (or 44 minutes and one cheilek; a cheilik equals three and one-third seconds, an 18th of a minute, or an 1080th of an hour). In the times of Chazal, based on eyewitness accounts of the New Moon, any month could have had 30 days (called a chodesh malei or full month) or 29 days (referred to as a chosesh chaseir). In our 19-year cycle Jewish calendar established by Hillel II (Hillel Nesiah; a thirteenth or fourteenth-generation descendant of Hillel Hazakein), every other month, alternates between 29 and 30 days. Whereas the Gregorian calendar maxim might be “Thirty Days has September, April, June, and November,” but in the Jewish calendar the colloquialism would probably be “Thirty Days has Tishrei, Shvat, Nissan, Sivan, and Av. Teves, Adar, Iyar, Tamuz, and Elul only have 29 days.”
An easy way to tell if the month you are in has 29 or 30 days is by seeing how many days of Rosh Chodesh the following month has. If it has two-days of Rosh Chodesh, that means the first day of Rosh Chodesh is actually the 30th day of the preceding month. For example, the first day of Rosh Chodesh Elul is actually Lamed Av. Even so, since it is connected to the month of Elul, there are halachos how the date is supposed to be written in official documents. Take our date of Lamed Av. Halachically it is to be written as “Rosh Chodesh Elul, Shehu Yom Shloshim L’Chodesh (which is the thirtieth of the month of) Av.” On the other hand, if it is a one day Rosh Chodesh, it is simply the 1st of that month.
As you probably realized, there were two months missing from the “adage of the months.” Marcheshvan and Kislev were not mentioned. The reason is that there is no hard and fast rule regarding them. They are the only months that can either have 29 or 30 days depending on the year. Some years both are chaseirim; other years both are malei’im, and others Cheshvan is chaseir and Kislev malei. The reasons and rules governing the whys and whens this transpires are too complicated for this discussion; suffice to say that when the Mishnah Berurah discusses these issues he writes “ain kan makom l’ha’arich,” that this is not the place to expound in detail, which is certainly good enough for this author. What is a given, is that this is one of the changing variables in our set calendar. Indeed, it is this specific variable that allows a Bar Mitzvah to occur before or after the actual presumed Bar Mitzvah date.
In fact, “Mein Alte Shachein”, the esteemed Bobover Rebbe, Rav Shlomo Halberstam zt”l, was known to have remarked that he personally started laying Tefillin as a Bar Mitzvah Bachur one day prior to his Bar Mitzvah. His intent was not that the Bobov minhag was to prepare for a Bar Mitzvah by laying Tefillin only one day before the actual Bar Mitzvah, but rather that he himself was full fledge “Bar Mitzvah-ed” (in the vernacular) before the date of his Bar Mitzvah. The reason why is quite interesting, due to a cosmic “quirk” in our calendar.
Will the Real Adar Please Stand Up?
But, this is not the only time that a calendar quirk changes a Bar Mitzvah. It is widely known that adding a leap year into the mix always has interesting Bar Mitzvah ramifications. The majority consensus is that if a boy was born in a non-leap year, one which there was only one Adar, and on the year of his Bar Mitzvah there are two Adars, his Bar Mitzvah will occur in the second Adar, since it is considered the true one. The same holds true if was actually born in Adar Sheini. In fact the only way one would celebrate a Bar Mitzvah in the first Adar is if he was actually born in an Adar Rishon.
This makes for a remarkable dichotomy. If one boy is born on the 21st of Adar Rishon, and his buddy a week and a half later on the 2nd of Adar Sheini, then in any standard year following, the second one would be celebrating his birthday almost 3 weeks before his “older” friend. Since there is only one Adar, the second born’s birthday would be the 2nd of Adar while his “older” friend’s would be on the 21st. In fact, only in a leap year would the older one truly be considered older. This would also affect their Bar Mitzvahs. If their Bar Mitzvah is in a standard year, the younger lad would become a man several weeks before his older compadre. However, it is important to note that this Klal may not hold true regarding Yahrtzeits.
Back to Cheshvan and Kislev. As detailed in recent articles, our current year, 5781 is a ZaCH”A זח"א)) year. This abbreviation is referring to Rosh Hashana falling out on Shabbos (zayin), both months of Cheshvan and Kislev being choseir (ches; 29-day months instead of 30), and Pesach falling out on Sunday (aleph).
I am sure that many noticed that the Molad (actual time of the ‘New Moon’ as it appears over Yerushalayim) stated on Shabbos Mevorchim Chodesh Marcheshvan was “Emesh, “yesterday,” (meaning Friday) for a Sunday-Monday Rosh Chodesh. This means that the actual lunar Rosh Chodesh started over a day and a half prior to when Rosh Chodesh is halachically observed. This also meant that the Sof Zman for reciting Kiddush Levana for the month of Cheshvan was earlier that the assumed 15th of the month.
Yet, as in our ZaCH”A year there is the rare double chodesh choseir, or three 29-day months in a row (Cheshvan, Kislev, and Teves) this serves to help synchronize the Molad and Rosh Chodesh, as each 29-day month brings them roughly half a day closer together.
Skip the Birthday
All of this background dramatically affects Bar Mitzvah bachurim. You see, if a boy was born on the 30th of Cheshvan - which is also the first day of Rosh Chodesh Kislev, in a year that Cheshvan was malei, and in his Bar Mitzvah year Cheshvan is a chaseir, meaning there is no 30th of the moth, our little lad does not become a man until the 1st of Kislev. In other words, he does not truly have a birthday, as in that year his birthdate simply does not exist. Hence, his birthday is skipped and is pushed off one day until the next month. He may have been born in Cheshvan, but his Bar Mitzvah will assuredly be observed in Kislev.
The reason for this is is that halachically one cannot become truly a Bar Mitzvah until he completes 13 entire years. Since there is no 30th of Cheshvan in his Bar Mitzvah year (this year), he does not actually reach that milestone until the next day, which is the one-day Rosh Chodesh Kislev, a.ka. Alef Kislev.
Bar Mitzvah-ed a Day Early
Yet, it is when the flip side of this equation occurs when it gets real interesting. The Elyah Rabba, based on a ruling of the Bach, maintains that if a boy is born on the 1st of Kislev in a year when Cheshvan only had 29 days, and in his Bar Mitzvah year Cheshvan has 30 days, then the boy becomes Bar Mitzvah on the first day of Rosh Chodesh Kislev, which is actually the 30th of Cheshvan! Since he completes 13 full years on that day (as there now is an extra day added to that year), he is obligated in Mitzvos on the day prior to his birthday. His birthday might be Alef Kislev, but his Bar Mitzvah is Lamed Cheshvan.
Although the Birur Halacha opines that it might still be preferable not to count this boy for a minyan until his actual birthday, and the rule would not apply for Yahrtzeits, nevertheless, this fascinating, albeit relatively obscure psak of becoming a Bar Mitzvah one day before the birthday is actually cited as halacha by the consensus of many great Acharonim, including the Shaarei Teshuva, Derech Hachaim, Rav Akiva Eiger, Pri Megadim, Machatzis Hashekel, Levushei Srad, Mishnah Berurah, and Kaf Hachaim. In fact, Rav Shmuel Halevi Wosner explicitly ruled that “lema’aseh hachi nahug.”
This is why the great Bobover Rebbe zt”l claimed he became Bar Mitzvah one day before his Bar Mitzvah date. He was born on Alef Kislev in a year when Cheshvan was chaseir (5668), and in his Bar Mitzvah year (5681) Cheshvan was malei.
Skip a Full Month?
Yet, in our exceptional year, there may even be a skip month for a Bar Mitzvah boy, and he perhaps may even celebrate two Bar Mitzvahs!! But we will have to wait several months for this uncanny occurrence to actualize. In fact, according to the Birur Halacha, the statistics of this rare phenomenon is an astounding 1:1,735 or .05763 percent.
The reason for this possibility is that thirteen years ago, in 5768, there was a 30 Adar I, which is not extant at all in our year, as 5781 only contains a single 29-day month of Adar. Hence, there is much confusion as to the date upon which a lad born on that day actually becomes Bar Mitzvah this year. Interestingly, there does not seem to be much discussion on this exact topic in the classic commentaries, and as it not a simple question, there are several options raised by recent authorities: 30 Shvat (1st day of Rosh Chodesh Adar), 29 Adar (Erev Rosh Chodesh Nissan), or 1 (Rosh Chodesh) Nissan.
Classically, the only posek prior the 1900s who ruled that such a Bar Mitzvah should be observed on 30 Shvat is the Aruch LaNer - due to his shittah of both days of Rosh Chodesh being considered a Yoma Arichta - one extended day. However, aside from Rav Shmuel Salant and the Maharil Diskin writing strongly against this shittah, it is simply not cited at all as part of most later poskims’ calculations on the topic. There are those, such as the Shevet Halevi, who write that it may be worthwhile to be machmir for his shittah [i.e. start wearing Tefillin earlier] but that the Aruch LaNer’s psak is certainly not obligatory.
There is the shittah of the Bais Shlomo who maintains in months where there is Adar confusion to start wearing Tefillin 31 days before the Bar Mitzvah, out of deference to the Maharash Halevi’s minority position that all observances are kept in Adar Rishon; but not that this shittah is the obligatory. As many authorities maintain to follow this shittah, this would certainly hold true in this case, which would mean that accordingly, our bachur should don Tefillin from 29 Shvat (Erev Erev Rosh Chodesh Adar).
When this exact sheilah was [astoundingly seemingly first] raised in the 1980s, most poskim, including the Ponovezher Rosh Yeshiva, Rav Elazar Menachem Mann Shach, the Steipler Gaon, Rav Shlomo Zalman Auerbach, Rav Moshe Sternbuch , Rav Yechezkel Roth, the Siach Halacha, and the Birur Halacha maintained that the ikar Bar Mitzvah should be observed on Rosh Chodesh Nissan. This is because the basic halacha is that Adar Sheini is the main Adar and if this young man, who was born at the end of Adar Rishon would become Bar Mitzvah on 30 Shvat, he would not have observed a month of Adar in his thirteenth year before becoming Bar Mitzvah. Ergo, only after he reaches a date similar to the last day of Adar this year, can he truly be considered Bar Mitzvah. Hence, his Bar Mitzvah would be on Rosh Chodesh (1) Nissan.
Conversely, the Minchas Yitzchak, Rav Yisrael Yaakov Fischer, the VaYaan Dovid of Antwerp, and the Pupa Rabbanim (the Vayaged Yaakov,Vayaan Yosef, andMishpatecha L’Yaakov; due to holding akin to the Aruch LaNer’s chiddush of both days of Rosh Chodesh being considered as one), held that 30 Shvat is the correct Bar Mitzvah date for our bachur, as although still in the month of Shvat, it is the first day of Rosh Chodesh Adar, and hence should has the status of Rosh Chodesh Adar, which was (sort-of) his birthdate (as 30 Adar Rishon is the first day of Rosh Chodesh Adar Sheini). They maintain that this should be similar to the halacha regarding Yahrtzeit observance, that one who was niftar on 30 Adar Rishon (first day of Rosh Chodesh Adar Sheini), his Yahrtzeit would be observed on 30 Shvat (first day of Rosh Chodesh Adar) in a non-leap year.
On the other hand, as Ashkenazim differentiate between Yahrtzeit and Bar Mitzvah observance, the opposing Poskim do not agree with this precedent’s relevance to Bar Mitzvah. In fact, even regarding a 30 Adar I Yahrtzeit, Rav Moshe Feinstein argued that it should be observed in Adar Sheini; hence the implications are that he would also maintain to push off the Bar Mitzvah’s observance until after Adar as well (to Rosh Chodesh Nissan).
There is also third opinion, that Rav Chaim Kanievsky is quoted as agreeing with (although only in sevara) - that 29 Adar should be the correct date, similar to the psak of being Bar Mitzvah-ed a day earlier due to a leap year on Rosh Chodesh Cheshvan, as discussed previously.
Ground Rule Double: Two Bar Mitzvahs Are Better Than One
Come what may, due to the uncertainty and machlokes, several poskim, including Rav Shlomo Zaman Auerbach [who apparently did not want to publicly be machria] and Rav Moshe Sternbuch, ruled to be machmir both ways - i.e. the Bar Mitzvah bachur should wear Tefillin and observe all of his obligations beginning 30 Shvat – the first day of Rosh Chodesh Adar [lechumrah], while other privileges, such as davening for the amud or counting in a minyan, will only begin on Rosh Chodesh Nissan. It is reported that Rav Yosef Shalom Elyashiv ruled that in such an eventuality, due to the machlokes, the bachur should even have two separate Bar Mitzvah Seudos.
Practically speaking, most Bar Mitzvah bachurim in this situation will follow this “split-up” approach - with two Bar Mitzvah dates a full month apart, but it seems that according to many poskim, this klal is M’Toras Chumrah, while the ikar Bar Mitzvah date would truly be 1 Nissan.
So, although not applicable with Rosh Chodesh Kislev this year, nonetheless, there is still quite a fascinating Bar Mitzvah push-off relevant this year; it is just regarding Chodesh Adar.
The Gemara stresses the importance of knowing the calculations of our calendar, with many Rishonim understanding that there is a specific mitzvah to do so. The Chazon Ish explicitly mentions the calculations of the Lunar month as a prime example of this. Now, at least we can all say that we are one step closer.
The author wishes to thank R’ Shloime Lerner,whose Mareh Mekomos served as the impetus for this author’s interest and research in this topic.
This article was written L’iluy Nishmas Shoshana Leah bas Dreiza Liba and l’zechus for Shira Yaffa bas Rochel Miriam v’chol yotzei chalatzeha for a yeshua sheleimah teikif u’miyad!
For any questions, comments or for the full Mareh Mekomos / sources, please email the author: firstname.lastname@example.org.
Rabbi Yehuda Spitz, author ofM’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 currently writes a contemporary halacha column for the Ohr Somayach website titled “Insights Into Halacha.” His first English halacha sefer, focusing on the myriad halachos related to food, is currently at the printer and is due out shortly.
 Rabban Gamliel’s statement (‘mekublani m’bais avi abba’) in Gemara Rosh Hashana (25a). See Rambam (Hilchos Kiddush HaChodesh Ch. 6:2 and 3), the Tur’s Seder HaKvius (Orach Chaim 427-428), the Mishnah Berurah’s Seder Laasos Luach B’Ketzara (end Orach Chaim 428), and Chazon Ish (Orach Chaim 138:4). This comes out to 29.53059 days. According to ‘Discovery’ by Rabbi Shraga Simmons (pg. 31), and Hamehapeich (by Rav Zamir Cohen; vol. 4, pg. 16-17), this data has recently been corroborated by NASA: “After years of research based on calculations using satellites, hairline telescopes, laser beams, and super computers, scientists at NASA have determined that the length of the ‘synodic month’, the time between one new moon and the next, is 29.530588 days.”
 See Ra’ah (Beitzah 4b) and Sefer Hachinuch (Parashas Bo, Mitzvah 4 – Kiddush Hachodesh).
 This is in a regular year; in a leap year Adar Rishon would have 30 Days and Adar Sheini would have 29.
 See Biur Halacha (428:1, end s.v. eilu hayamim).
 This wording follows the Tur and Shulchan Aruch’s opinion (Orach Chaim 427:1); this is also how the Aruch Hashulchan (ad loc.) concludes lemaaseh. However, the Bach (ad loc. s.v. v’kosvin) and Pri Megadim (ad loc. M.Z. 1) maintain that the order should be reversed and written as “Yom Shloshim L’Chodesh Av, Shehu Yom Rosh Chodesh Elul.” See Mishnah Berurah (ad loc. 1).
 Biur Halacha (428:1, end s.v. eilu hayamim). He also writes a tad earlier that “v’hinei kol zeh shekasavnu ain tzarich leha’arich b’frotrot aich hu kein, rak sheteida haklal,” (loosely) that all of these matters do not need to be measured in their exact minutiae, but rather one should know the general rules.
 Growing up in Miami Beach, one of the Bobover Rebbe’s prime Chassidim, Reb Yechiel Kurtz a”h maintained a vacation home directly across the street from us. He graciously allowed the Rebbe full use of his home, and the Rebbe zy”a would visit several times a year, often inviting us for Seudas Shlishis or Melaveh Malka. Years later, when visiting the Rebbe in New York, he would always greet us with a warm smile and referring to each of us as “Mein Alte Shachein.”
 See Gilyon Shoneh Halachos (Bobov; Chodesh Nissan 5771, pg. 11), in a letter from Rav Yehoshua Shalom Nosson Rubin. Thanks are due to R’ Shloime Lerner for providing this source.
 Rema (Orach Chaim 55:10; based on Shu”t Mahari Mintz 15), Levush (Orach Chaim 685:1), Magen Avraham (Orach Chaim 55:10), Pri Chodosh (ad loc. 10), Pri Megadim (ad loc. Eshel Avraham 10; he adds that m’pashtus this is also the Shulchan Aruch’s shittah), Levushei Srad (ad loc. s.v. eino), Korban Ha’eidah (on Yerushalmi Megillah Ch. 1, Shiyarei Hakorban s.v. hada), Shaarei Teshuva (Orach Chaim 55:11), Gilyon Maharsha (Yoreh Deah 402, s.v. b’Adar), Chayei Adam (vol. 1, 66:1), Kaf Hachaim (Palaji; 13:2), Kitzur Shulchan Aruch (15:2), Aruch Hashulchan (Orach Chaim 55:14), Aruch LaNer (Shu”t Binyan Tzion 151; he maintains that even if one was born on 30th of Shvat, a.k.a. the first day of Rosh Chodesh Adar in a standard year, gets pushed off until the first day of Rosh Chodesh Adar Sheini, as both days of Rosh Chodesh have the status of ‘Rosh Chodesh Adar’), Mishnah Berurah (ad loc. 45), and Kaf Hachaim (ad loc. 59); not like the Maharash Halevi (or Maharash L’Bais Halevi; Shu”t Orach Chaim 16) who was of the opinion that in such a case, all observances should be celebrated in Adar Rishon.
 Although the Magen Avraham (ibid., based on his understanding of the Mahari Mintz’s position) maintains that even a boy born in an Adar Rishon’s Bar Mitzvah gets deferred to Adar Sheini, and the Olas Hatamid (ad loc.) and Chasam Sofer (Shu”t Orach Chaim 163: end 3; although in Siman 14, he seems to contradict this position and rule accordingly to the majority consensus) agree with him, nevertheless, the consensus of poskim is that one who is born in an Adar Rishon’s Bar Mitzvah is observed in Adar Rishon as well; if he was born in a standard Adar or Adar Sheini, his Bar Mitzvah would be observed in Adar Sheini. These poskim include the Shulchan Aruch (ibid.), Levush (Orach Chaim 685 1), Pri Chodosh (ad loc. 10; citing the Yerushami Megillah Ch. 1:5, that Adar Rishon is merely a ‘tosefes’), Shvus Yaakov (Shu”t vol. 1:9; who writes that the Magen Avraham misunderstood the Mahari Mintz), Elyah Rabba (Orach Chaim 55:9 and Elyah Zuta 5), Rav Dovid Oppenheim (cited in the Ba’er Heitiv ad loc. 11), Me’il Tzadaka (Shu”t 21), Shaarei Teshuvah (Orach Chaim 55:11), Maamar Mordechai (ad loc. 13), Pri Megadim (ad loc. Eshel Avrohom 10), Ikrei Hadat (3:7), Chida (Machzik Bracha ad loc. 4), Machatzis Hashekel (ad loc. 10; he also questions the Magen Avraham’s understanding), Chayei Adam (vol. 1, 66:1), Kaf Hachaim (Palaji; 13:2), Maharsham (Daas Torah ad loc. s.v. u’shnas), Kitzur Shulchan Aruch (15:2), Aruch Hashulchan (Orach Chaim 55:14), Mishnah Berurah (ad loc. 43), and Kaf Hachaim (ad loc. 59). The Ba’er Heitiv (ibid.) concludes that “v’chein haminhag pashut eitzel kol ba’alei hahora’ah.” See also the Beis Yitzchok’s haskama to Rav Nosson Nota Landau’s Oorah Shachar, who as a side point, cites his ancestor the Maharam Padua as ruling this way as well. On an anecdotal level, this halacha affected this author growing up, as my birthday was Rosh Chodesh Adar and my Bar Mitzvah occurred on a leap year. This also affected my son who was born on my birthday as well, as his Bar Mitzvah was observed on Rosh Chodesh Adar Sheini 5776. [However, as an interesting counter-point, it had the opposite effect on his twin sister - as they were both born on Rosh Chodesh Adar Sheini and her Bas Mitzvah was on a standard Adar year. That means she reached the age of Mitzvos 13 months before her twin brother!]
 See footnote 21. This was discussed at length in a previous article titled “A Tale of Two Adars: Computations and Calculations – Bar Mitzvahs and Yahrtzeits.”
 This is because although one technically has up until half the month or half of 29 days, 12 hours, and 793 chalakim from that month’s Molad [see Tur, Shulchan Aruch and Rema (Orach Chaim 426:3; citing Shu”t HaMaharil 19) and Pri Chodosh (ad loc. 3); and although the Pri Megadim (ad loc. Eishel Avraham 13) grapples with whether or not this is truly a nuanced machlokes, or perhaps three distinct shittos, and the Mishnah Berurah (Biur Halacha ad loc.s.v. v’lo) implies this way as well, conversely, the Aruch Hashulchan (ad loc. end 11) understands the Shulchan Aruch and Rema to be agreeing, just expressing themselves using their own terminology], nonetheless, as this zman follows the Molad and its halfway point as they occur in Yerushalayim, in Chodesh Cheshvan, this was at least a day earlier than possibly presumed. This was discussed at length in a previous article titled “Kiddush Levana During the Megillah?!”.
 Thanks are due to Rav Dovid Heber, author of Shaarei Zmanim, for pointing this out.
 Magen Avraham (Orach Chaim 55: end 10 s.v. u’mi), Elyah Rabba (ad loc. 9), Levushei Srad (ad loc. s.v. d’naaseh), Derech Hachaim (Dinei Kaddish B’Asarah 4), Shulchan Aruch Harav (Orach Chaim 55:13), Shaarei Teshuva (ad loc. 13), Mishnah Berurah (ad loc. 45), Aruch Hashulchan (ad loc. 15), and Kaf Hachaim (ad loc. 60).
 See Shu”t HaBach (Hayeshanos 145), SM”A (Even Ha’ezer 35:2), Magen Avraham (Orach Chaim 55:10), Ba’er Heitiv (ad loc. 11), Elyah Zuta (ad loc. 4), and Mishnah Berurah (ad loc. 45).
 Thanks are due to R’ Eli Dov Stern for pointing out this phenomenon.
 Elyah Rabba (Orach Chaim 55: end 9) based on the Bach (ibid.), although he implies that to his understanding, the Magen Avraham (cited above, that even a boy born in Adar Rishon’s Bar Mitzvah is observed in Adar Sheini) will not necessarily agree with this. However, the Pri Megadim (ibid.) counters that in his opinion, the Magen Avraham would indeed agree to this ruling of the Bach et al.
 A potential additional reason for this ruling is that the Aruch LaNer (Shu”t Binyan Tzion 151) maintains that both days of Rosh Chodesh have a status of one day; meaning they are considered somewhat connected. [Although admittedly, he is referring to the opposite case – that one who was born on the 30th of Shvat (a.k.a. the first day of Rosh Chodesh Adar) in a regular year, if his Bar Mitzvah is in a leap year it should be observed a month later – on Rosh Chodesh Adar Sheini. Although this is not the halacha pesuka, and Rav Shmuel Salant and the Maharil Diskin ruled against it (as cited in Shu”t Shevet Halevi vol. 10:105), nonetheless, in such a situation Rav Wosner himself (Shu”t Shevet Halevi ibid.) and Rav Shlomo Zalman Auerbach (as cited in Ishei Yisrael Ch. 15, footnote 26) recommended being machmir and to wait until Rosh Chodesh Adar Sheini for Inyanei Deoraysa, i.e. for him to Daven for the Amud, be Motzi others’ obligations, or get an Aliyah other than Maftir etc. Of course he should still wear Tefillin etc. from his true (mainstream) Bar Mitzvah date]. According to this shittah, once our growing lad’s 13 years are complete and it is already Rosh Chodesh, he would already be considered (at least somewhat of) a Bar Mitzvah, even though his true birthday is the following day (of Rosh Chodesh). The Aruch Hashulchan (Orach Chaim 55: end 15) implies this way as well, and this logic is also cited lemaaseh by the Vayaged Yaakov (Shemos, Mitzvosecha Sha’ashu’ai, Mitzvah 4) and his son the Vayaan Yosef (Greenwald; Shu”t vol. 1:33, 1 s.v. v’heishiv), and grandson, the Mishpatecha L’Yaakov (Shu”t 13), and is mentioned in Ishei Yisroel (pg. 135, Ch. 15, footnote 26, in the parenthesis).
 Birur Halacha (Rav Yechiel Avraham Zilber; in his comments on the Mishnah Berurah 55:45, vol. 1, pp. 131-133). He posits that although our protagonist may take on the Mitzvos of a Bar Mitzvah bachur, it is nonetheless preferable to wait until his actual birthday to count him for a minyan or zimun, unless it is shaas hadchak. This author finds it interesting that his proofs are based on the dinim of yahrtzeit, which technically are not truly the same. As previously mentioned, and although the Piskei Teshuvos (vol. 1, ad loc. pg. 492:20), seemingly quite oddly opines to follow the Birur Halacha and not the hachraas poskei hadoros, nonetheless, as proven, this is not the normative halacha.
 An interesting side point is that according to the halachic consensus, this ruling would not apply to Yahrtzeits, which, in such a case, the majority of poskim rule should be observed on the second day of Rosh Chodesh. See Yoreh Deah (402:1), Magen Avraham (Orach Chaim 568:20), Taz (Yoreh Deah 65:8), Chochmas Adam (171:11), Matteh Efraim (Dinei Kaddish Yasom, Shaar 3:7), Elef L’Matteh (ad loc. 8), Kitzur Shulchan Aruch (221:4), Aruch Hashulchan (Orach Chaim 568:15), and Gesher Hachaim (pg. 345); not like the Shu”t Panim Meiros (vol. 2:83; cited in Shaarei Teshuva 568:17). The reason is that a Yahrtzeit is observed on the date when someone is niftar; therefore it has to be on that same date. Moreover, as Yahrtzeit observance is technically considered a neder (vow), the rule of thumb is that regarding nedarim, its observance follows the common vernacular. As Lashon Bnei Adam is to refer to the 1st of Kislev as Rosh Chodesh Kislev, that is the date that must be observed regardless of the month’s makeup. This is in contrast to when a child becomes Bar Mitzvah on the day he completes 13 full years. Therefore, a Bar Mitzvah technically does not have to be exactly on the actual birthday. This was discussed at length in a previous article titled “A Tale of Two Adars: Computations and Calculations – Bar Mitzvahs and Yahrtzeits.”
 Shaarei Teshuva (Orach Chaim 55:11), Derech Hachaim (Dinei Kaddish B’Asarah, end 4), Rav Akiva Eiger (Orach Chaim 55:10 s.v. v’am”sh), Pri Megadim (ad loc. Eishel Avraham 10), Machatzis Hashekel (ad loc. s.v. ad sheyavru), Levushei Srad (ad loc. s.v. ad sheyavru), Mishnah Berurah (ad loc. end 45), Kaf Hachaim (Palaji 13:2 and Sofer - Orach Chaim 55:60), Amudei Hashulchan (on Kitzur Shulchan Aruch 15:1), and Ishei Yisrael (pg. 135, Ch. 15, footnote 26, in the parenthesis). The Pri Megadim adds that the Magen Avraham would even be maskim to this. The Aruch Hashulchan also (ad loc. end 15) implies this way. The Vayaged Yaakov (ibid.) and his son, the Vayaan Yosef (ibid.) rule this way as well. Rav Wosner’s Teshuva ruling this way is printed in Kovetz Kol Torah (vol. 53, Mador Halacha, pg. 183–184).
 Birur Halacha (Tinyana pg. 137:10).
 Indeed, the Shulchan Aruch (Orach Chaim 55:10) only discusses what to do when a boy born on 29 Adar I in a leap year becomes Bar Mitzvah in a non-leap year, but does not discuss 30 Adar I at all.
 Shu”t Binyan Tzion (151); as cited previously.
 See (Shu”t Ben Shmuel 6) and Shu”t Shevet Halevi (vol. 10: 105, 2).
 Shu”t Shevet Halevi (vol. 6:9 and vol. 10: 105, 2). The Karlsburger Rav, Rav Yechezkel Roth (Shu”t Eimek HaTeshuva vol. 2:5) writes similarly, that the halacha does not follow this shittah of the Aruch LaNer.
 Shu”t Beis Shlomo (Even Ha’ezer 56)andShu”t Maharash Halevi (Shu”t Orach Chaim 16). Although not everyone follows his chiddush, it is cited by many poskim l’maaseh. See Shu”t Shevet Halevi (vol. 6: 9 and vol. 10: 105, 2), Shu”t Tzitz Eliezer (vol. 13:10 s.v. ela and u’chyadua), Moadim U’Zmanim (vol. 7: 250 s.v. v’chol zeh and u’lefi aniyus daati), Orchos Chaim (Spinka; 37), Igros HaKodesh (5717; vol. 14: 243, postscript), and Shu”t Lehoros Nosson (vol. 12:5). Thanks are due to R’ Shloime Lerner for pointing out and providing several of these invaluable sources. This was discussed at length in a previous article titled “31 Days Before the Bar Mitzvah: A Primer on Mitzvah Observance in a Double Adar.”
 The shittos of all three aforementioned poskim is cited in Birur Halacha (vol. 4, pg. 49-50).
 Moadim U’Zmanim (vol. 7:250, footnote 1), Shu”t Eimek HaTeshuva (vol. 2:5), Siach Halacha (vol. 2:55, 15), and Birur Halacha (ibid.).
 Shu”t Minchas Yitzchak (vol. 6:8 and vol. 8:7; however, he does conclude both teshuvos stating “d’lekula adayin yeish lehisyasheiv”), Shu”t Even Yisrael (vol. 9:63, on O.C. 55:10), Shu”t VaYaan Dovid (vol. 1:11, 4), the Vayaged Yaakov (Shemos, Mitzvosecha Shaashu’ai, Mitzvah 4), Shu”t Vayaan Yosef (vol. 1:33), and Shu”t Mishpatecha L’Yaakov (13). The Yalkut Yosef (Kitzur Shulchan Aruch, Hilchos Purim, Mei’hilchos Chodesh Adar 9) implies this way as well, first citing “yeish lehistapeik” which side is correct, and then concluding that the Mishnah Berurah (568:42), regarding the halacha of a Yahrtzeit, is “mashma shenaaseh k’gadol B’Alef D’Rosh Chodesh Adar, shehu Lamed Shvat.”
 See Magen Avraham (Orach Chaim 568:20), Machatzis Hashekel (ad loc.), Pri Megadim (S.D. ad loc. 20), Chochmas Adam (171:11), Aruch Hashulchan (Orach Chaim 568:15), Mishnah Berurah (ad loc. 42), and Kaf Hachaim (ad loc. 86). This is following most later authorities’ [including the Chavas Daas’ Mekor Chaim, Rabbi Akiva Eiger, the Yad Efraim, and Levushei Srad (ad loc.)] slight amendments to the Magen Avraham’ text (seemingly first pointed out in the Kuntress Acharon of the Tosefes Shabbos) due to printing error. Indeed, the Elyah Rabba (Orach Chaim 685: end 7) calls the implications of the original wording of Magen Avraham’s (that the Yahrtzeit should be observed on 29 Shvat) a “temuah.”
 Rema (Orach Chaim 568:7 and Yoreh Deah 402:12). See Levush (Orach Chaim 685: 1) for a detailed explanation. See also Shu”t Har Tzvi (Orach Chaim vol. 2:83). This differentiation between Yahrtzeits and Bar Mitzvahs was touched upon previously, and discussed at length in a previous article titled “A Tale of Two Adars: Computations and Calculations – Bar Mitzvahs and Yahrtzeits.”
 Shu”t Igros Moshe (Y.D. vol. 3:159, 4).
 Cited in Birur Halacha (ibid.), quoting his friend, Rav “Chaim Chaikel B.”
 Moadim U’Zmanim (ibid.). Rav Shlomo Zalman’s and Rav Elyashiv’s “Double Bar Mitzvah” shittah is cited in Birur Halacha (ibid. pg. 49-50) and Ishei Yisrael (Ch. 15, pg. 135, footnote 26). In Shu”t Vayaan Dovid (ibid.) the author reports that the Satmar Rebbe replied similarly, that the bachur should receive the Bar Mitzvah Maftir Aliyah twice, once the week of 30 Shvat-Rosh Chodesh Adar, and then a month later, on the week of Rosh Chodesh Nissan.
 Shabbos (75a).
 They are divided though, whether it is a Mitzvah Deoraysa or Derabbanan. The BeHa”G (Mitzvos Kum Asei 75), SMa”G (Asein 47), SMa”K (Mitzvah 103; however, he holds that this is part of the Mitzvah of Kiddush Hachodesh) and the Sefer Yereim (Mitzvah 60), count this as a Mitzvah Deoraysa, while the Rambam in his Sefer HaMitzvos (Shoresh HaRishon) and the Ramban (Hasagos ad loc.) write explicitly that they are are of the opinion that it is Derabbanan.
 The Chazon Ish (Orach Chaim 138:4) writes that knowing the calculations for the lunar month and other Tekufos is considered “Torah.” See also Rav Chaim Kanievsky’s introduction to his Shekel HaKodesh, where he recounts the importance and reverence the Chazon Ish held of calculating the months, and how he even would consider his mathematical calculations as “Geniza.” See Rav Yisroel Reisman’s excellent Pathways of the Prophets (pp. 272–311), who expounds upon and elucidates practical examples of understanding calendar calculations at length.