As we already learned (6:1), the prophets instituted four fasts after the destruction of the First Temple, the gravest of which is Tisha Be-Av, the ninth day of the Hebrew month of Av, the day that the Temple was destroyed (for more on the meaning of the fast, see above 6:4). These fasts were modeled on Yom Kippur, which lasts an entire day and contains five prohibitions. When the Second Temple was built, these fasts were annulled, becoming joyous holidays. And when the Second Temple was destroyed, the four fasts reverted to their original status.

After the period of harsh decrees ended and a new era began, in which the Temple was still in ruins but evil decrees no longer plagued us, the Sages determined that the status of three of the fasts – Asara Be-Tevet, Shiv’a Asar Be-Tamuz, and Tzom Gedalia – depended on the will of the Jewish people: “If they want, they fast; if they do not want, they do not fast.”

On Tisha Be-Av, however, we must fast under all circumstances, because many tragedies befell the Jewish people on that day, including the destruction of both Temples. Even when we live in a tranquil time, its status is not dependent on Israel’s will. Rather, as long as the Temple sits in ruins, we must fast on that day, as the prophets enacted (rh 18b).

This is the basis for the difference between Tisha Be-Av and the minor fasts. Our obligation to fast on Tisha Be-Av is based on an institution of the prophets, so all the laws of the fast apply in their entirety. On the minor fasts, however, our obligation is based on custom, since all of Israel agreed to fast on these days until the Temple is rebuilt. Thus, from the very outset, the accepted custom has been to treat them more leniently than Tisha Be-Av (as explained above, 7:1).[1]