This Tuesday begins the three weeks. From the 17th of Tamuz until the 9th of Av is a time of mourning for Klal Yisrael. We have accepted upon ourselves certain customs of mourning. We don’t get married, listen to music with musical instruments, get haircuts, or recite a “Shehechiyanu”.
The three weeks commence with the fast on the 17th of Tamuz. A commomly asked question is when the customs of the three weeks take effect. For example, if someone remembered on the night before the fast that he wants a haircut, is it too late? Or maybe the prohibition only starts in the morning when the fast starts. The Mishna Berurah 550:2 says in the name of the Shlah that even though we have the custom of fasting only from the morning, it is meritorious for one to fast from nightfall. This is based on the reasoning of early commentators (Ramban, Ran) that the reason we fast from the morning is because it is too hard for most people to start from the night. It would seem though, that if an individual can fast from the night, he should. Based on this, it is clear that the prohibitions of the three weeks would start from the night, only regarding the fast are we lenient. See however Igros Moshe 1:168 and 4:112:2 that Rav Moshe Feinstein was lenient if the fast fell on a Sunday or in a situation of urgency.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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