As we already learned (6.1), the prophets instituted four fasts after the destruction of the First Temple, the gravest of which is Tish’a B’Av, for that is when the Temple was destroyed (for more on the meaning of the fast, see above, 6.4). These fast were instituted along the lines of Yom Kippur, which lasts an entire day and contains five prohibitions. When the Second Temple was built, these fasts were annulled, and they became joyous holidays. And when that Temple was destroyed, the four fasts returned 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 law of three of the fasts – the tenth of Tevet, the seventeenth of Tammuz, and Tzom Gedalyah – depended on Israel’s will: “If they want to fast, they do so; if they do not want [to fast], they do not fast.”
On Tish’a B’Av, however, we are obligated to fast under all circumstances, because many tragedies befell the Jewish people on that day, including the destruction of both Temples. Even when times are tranquil, 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 (Rosh HaShanah 18b).
This is the fundamental difference between Tish’a B’Av and the minor fasts. Our obligation to fast on Tish’a B’Av is based on an institution of the prophets; therefore all the laws of the fast apply in their entirety. On the minor fasts, however, our obligation is based on custom – because [all of] Israel agreed to fast [on these days], until the Temple is rebuilt – and from the very outset, the accepted custom was to treat them more leniently than Tish’a B’Av (as we explained above, 7.1).