- Peninei Halakha
There are three halakhic differences between Tisha Be-Av and the minor fasts: 1) Tisha Be-Av lasts an entire day, from shki’at ha-ĥama (sunset) of one day until tzeit ha-kokhavim (the emergence of three mid-sized stars) of the next day, whereas the minor fasts last only during daylight hours – from alot ha-shaĥar (dawn) until tzeit. 2) On Tisha Be-Av, there are five prohibitions: a) eating and drinking; b) washing; c) anointing oneself; d) wearing high-quality shoes; and e) engaging in sexual relations. On the minor fasts, however, only eating and drinking are prohibited. 3) Only sick people are exempt from fasting on Tisha Be-Av, whereas on the minor fast days, pregnant and nursing women are exempt as well.
In general, the fast of Tisha Be-Av is equivalent to the fast of Yom Kippur: all enactments the Sages made regarding Tisha Be-Av were modeled after the Torah’s commandments regarding Yom Kippur. This includes the five prohibitions listed above. Nonetheless, since the fast of Tisha Be-Av is a rabbinic enactment, its laws are more lenient in two significant ways: 1) The Sages did not require sick people to fast on Tisha Be-Av. In contrast, sick people must fast on Yom Kippur, unless doing so would put his life in danger. 2) In situations of uncertainty, one should be stringent on Yom Kippur, but on Tisha Be-Av one may be lenient. This follows the rule that we are stringent in cases of uncertainty about Torah law, but lenient in cases of uncertainty about rabbinic law.
However, Tisha Be-Av is more stringent than Yom Kippur in a certain way. On Tisha Be-Av, we are obligated to mourn. Therefore, we sit on the floor, refrain from greeting one another, darken the room at night, and are forbidden to enjoy Torah study. On Yom Kippur, however, we are obligated only to deprive ourselves, but besides the five afflictions, the day is considered a holiday: We wear nice clothing, sit on regular seats, sing songs, exchange greetings with one another, and of course, engage in Torah study without limits. (Regardless of the afflictions, melakha is forbidden on Yom Kippur just as it is forbidden on Shabbat.)