850. The Differences Between Tisha b'Av and Other Fasts

121:7 If one of the four fasts falls during one's week of sheva brachos, the couple must fast even though sheva brachos is considered a personal festival. This is because the fasting of the community supersedes the festival of an individual as per Psalms 137:6, "If I do not set Jerusalem above my highest joy." (This law applies even when a fast is pushed off from Shabbos to Sunday - Bi'ur Halacha 559:9 s.v. v'nidcheh.)

121:8 There is a difference between the first three fasts and Tisha b'Av. On the first three fasts, we may eat on the night before until daybreak so long as one did not go to sleep for the night; if one did sleep the night (even not in a bed - Mishnah Brurah 564:3), it is forbidden to eat or drink later unless he made a condition to that effect before going to bed. If one is accustomed to drink after sleeping, he need not make a condition about drinking. On Tisha b'Av, however, one must stop eating and drinking on the day before while it is still daytime. Also, on the first three fasts, we are permitted to wash, anoint, wear leather shoes, and engage in marital relations, while these things are prohibited on Tisha b'Av. One who is scrupulous in mitzvah observance and in good health should be strict in these matters on all of the fasts like he is on Tisha b'Av except for not wearing leather shoes. One should not do this on other fasts because it is arrogant (that he is visibly acting more stringently than halacha requires). If it is his wife's mikvah night, one should have marital relations on the first three fasts. (If 10 Teves falls on a Friday, one should not be strict about washing with hot water, out of respect for Shabbos - MB 550:6.)