The day of Shiba Asar Be'Tammuz (the 17th day in the month of Tammuz) is observed as a fast day to commemorate five calamities that befell the Jewish people on this day: 1) Just several months after the Exodus from Egypt, Beneh Yisrael worshipped the golden calf on Shiba Asar Be'Tammuz; 2) Toward the end of the First Temple era, it was on this day when the daily Tamid offering was discontinued; 3) Just before the destruction of the Second Temple, the wall of Jerusalem was breached by the Roman legions on the 17th of Tammuz; 4) An enemy named Apostomus publicly burned a Sefer Torah; 5) A statue was brought into the Bet Hamikdash. Other calamities occurred on this day, as well. The Gemara in Masechet Ta'anit establishes the rule of "En Ta'anit Sibur Be'Babel." This means that when the Sages in Babylonia established fast days, they did not treat them with the same severity as Yom Kippur and Tisha B'Ab. Thus, it is permissible to wash, bathe and wear leather shoes on Shiba Asar Ba'Tammuz. Furthermore, the fast begins in the morning, and not at sundown the previous evening. The Halachic authorities address the question of whether or not the restrictions of "Ben Ha'mesarim" – the three-week period between Shiba Asar Be'Tammuz and Tisha B'Ab – apply on the night before Shiba Asar Be'Tammuz. The period of the Three Weeks is a time when many calamities befell the Jewish nation, in commemoration of which we observe a number of prohibitions. Do these restrictions take effect on the morning of Shiba Asar Be'Tammuz, when the fast begins, or do they take effect already the previous night? The Hid"a (Rav Haim Yosef David Azulai, 1724-1806) addresses this question in his work of responsa Haim Sha'al, and he writes that one should begin observing the restrictions of the Three Weeks already on the night before Shiba Asar Be'Tammuz. Other authorities rule leniently in this regard, allowing haircutting and festivity on the night before Shiba Asar Be'Tammuz, and therefore those who are lenient have a basis on which to rely. Preferably, however, one should refrain from such activities already at the night of Shiba Asar Be'Tammuz, except in situations of a particular need. It should be noted that according to some authorities (including the Ramban and the Shela), the fast of Shiba Asar Be'Tammuz begins already the previous night. Of course, Halacha does not follow this opinion, but it nevertheless demonstrates that the night of Shiba Asar Be'Tammuz is considered the beginning of the Three Week period, and it is therefore proper to begin observing the restrictions of the Three Weeks already on this night. Specifically, these restrictions take effect at Set Hakochavim (nightfall) of the evening of Shiba Asar Be'Tammuz. Rav Yosef Shalom Elyashiv (contemporary) rules that since these restrictions are of Rabbinic (as opposed to Biblical) origin, they do not have to be observed during Ben Ha'shemashot on the evening of Shiba Asar Be'Tammuz. It is only at nightfall, when the stars become visible, that one must begin observing the restrictions of the Three Weeks. Summary: The restrictions of the Three Weeks begin at nightfall the evening before Shiba Asar Be'Tammuz. Even though the fast of Shiba Asar Be'Tammuz does not begin until morning, one should begin observing the laws of the Three Weeks already at nightfall the previous night, except in situations where it is necessary to delay these observances until morning.