When Do The Three Weeks Start?
We are currently in the period of mourning of “Bein HaMetzarim1”, or ‘Between the Confines (Straits)’. This period of Three Weeks commemorates the tragedies that took place prior to the destruction of both Batei Hamikdash, from the breaching of the walls of ancient Jerusalemon the 17th of Tammuz, until the actual destruction of the Beis HaMikdash on the 9th of Av. Both of these days have since become communal Fast Days, in remembrance of the tragedies that happened on these days2.
In order to properly commemorate and feel the devastation, halacha mandates various restrictions3 during these “Three Weeks”, getting progressively stringent up until Tisha B’Av itself4. These restrictions include not getting married, not getting haircuts, refraining from refraining from public music and dancing, not putting oneself in an overly dangerous situation, and not making the shehechiyanu blessing on a new item (i.e. refraining from purchasing a new item on which one would be required to make the blessing).
There is some debate in recent Rabbinic literature as to when the prohibitions of the ‘Three Weeks’ start. I was actually asked this sheilah a few times during this past week:
“Rabbi, I know the Three Weeks technically start tonight5, but since the Fast of 17th of Tammuz only starts in the morning, can I still get a haircut and/or shave this evening?”
Rav Moshe Feinstein addressed a similar question over sixty years ago: whether one may get married on the night of the 17th of Tammuz. He maintains that since 1) there is some debate among the early authorities as to whether the restrictions depend on the fast day itself6, 2) the whole issue is only a minhag to show communal mourning7, and 3) a wedding is considered l’tzorech (a great need), one may be lenient and get married on the eve of the 17th of Tammuz.
Some poskim extrapolate that Rav Moshe would hold similarly concerning a haircut - that if there is great need, then one may be lenient as well.
However, Rav Shmuel Wosner disagrees with this theory and maintains that regarding a wedding (especially on Motzei Shabbos, which actually was the original question asked of Rav Moshe) there is more halachic rationale to rely upon than for a haircut. Furthermore, haircuts are generally not considered "a great need".
Interestingly, years later, Rav Moshe addressed this issue directly and maintained that the same leniency does apply to haircuts and one may therefore get a haircut on the eve of the 17th of Tammuz in times of great need (and not as Rav Wosner understood Rav Moshe’s opinion).
Nevertheless, many contemporary halachic decisors, including Rav Wosner himself, as well as Rav Shlomo Zalman Auerbach, Rav Yosef Shalom Elyashiv, and Rav Nissim Karelitz, feel that the issue is moot, and that even for a wedding, let alone a haircut, one should not exercise leniency, as the eve of the 17th is already considered part and parcel of the ‘Three Weeks’, and thus is included in the restrictions.
So, even if one feels he desperately needs a haircut (perhaps someone suffering from lycanthropy) on the 16th of Tammuz, it is definitely preferable to go to the barber right away and not wait until evening, thereby avoiding subjecting oneself to a halachic dispute.
However, it is important not to lose sight of the forest for the trees. Instead of debating the finer points of whether a haircut is permitted or forbidden, it is important for us all to remember that these restrictions were instituted by our Rabbis as a public show of mourning during the most devastating period in the Jewish year. As the Mishna Berura explicitly notes, these days of sorrow and mourning serve to remind us of the national tragedies that befell our people, and the events that led to them. Our goal should be, then, to utilize these restrictions to focus inward, on our own personal challenges in our relationship with G-d, and rectify that negativity which led to these tragic events in our history.
 This three-week season is referred to as such by the Midrash Rabbah (Eicha 1:3).
 See Mishna in Maseches Ta’anis 26b and accompanying Gemara.
 This is following Ashkenazic minhag; many Sefardim only start restrictions on beginning of the week that Tisha B’Av falls out on.
 See Shulchan Aruch, Rema and their commentaries O.C. 551.
 As in Judaism, the start of a halachic new day is the preceding evening.
 Meaning that if the ‘Three Week’ restrictions are dependant on the Fast of the 17th of Tammuz, then they would only start at the same time the fast does - on the morning of the 17th. But if they are considered independent of each other, then the restrictions would start on the preceding evening, even though the fast itself would only start the next morning.
 Communal mourning is only recognizable in the morning when everyone is fasting.
Disclaimer: These are just a few basic guidelines and overview of the Halacha discussed in this article. This is by no means a complete comprehensive authoritative guide, but rather a brief summary to raise awareness of the issue. One should not compare similar cases in order to rules in any real case, but should refer his questions to a competent Halachic authority.