When Tish’a B’Av falls out on Shabbat, it is postponed until Sunday. On that Shabbat, we show no [outward] signs of mourning; rather, we eat and drink like we do on any other Shabbat, even feasting like King Shlomo did in his day (see above 9.4).
As we learned above (10.4), pregnant and nursing women must fast on Tish’a B’Av. On a postponed fast, however, the law is more lenient, and if they feel slightly weak or if they experience some type of pain, they are exempt from fasting, even though they are not [actually] ill (B.H. 559:9, s.v. ve’eino; K.H.C. 75).
The main participants of a brit milah [the father, the mohel, and the sandak] must fast on Tish’a B’Av, but the law is more lenient when Tish’a B’Av falls out on Shabbat and the fast is postponed until Sunday. According to most poskim, these participants may pray Minchah after midday, perform the circumcision immediately afterwards, and then eat and drink. Some poskim rule strictly on the matter. In practice, the prevalent custom is to perform the brit towards the end of the day and eat the meal after the stars emerge.
When Tish’a B’Av falls out on Shabbat and the fast is postponed until Sunday, the tenth of Av, the customs of mourning do not continue after the fast, and one is allowed to take a haircut, do laundry, and bathe in hot water once the stars emerge. However, many [authorities] maintain that one should refrain from eating meat and drinking wine that night. Since everyone fasted during the day, it is improper to immediately rejoice by consuming meat and wine (Rama 558:1, M.B. 4-5, Rav [Mordechai] Eliyahu’s Hilchot Chagim 29:9). Others permit the consumption of meat and wine immediately following the fast (R. Chayim Vital, Pri Chadash, Torat HaMo’adim 11:8).
. The Shulchan Aruch (554:19) writes that when Tish’a B’Av coincides with Shabbat, one is permitted to do all [that is ordinarily forbidden to be done on Tish’a B’Av], even marital relations. The Rama, however, says that one should refrain from such relations, for [acts that constitute] private mourning remain forbidden [on that Shabbat], because [refraining from them] does not diminish the honor of the Sabbath. The Mishnah Berurah (40) writes – based on the Shelah and the Magen Avraham – that if a woman goes to the mikvah [that Friday night], thus making it a mitzvah to have marital relations, then even those who [usually] follow the Rama’s customs would rely on the Shulchan Aruch’s opinion.
. The Shulchan Aruch (559:8) rules leniently and permits [the three main participants] to partake in a meal in honor of the brit on a postponed fast day. Other [poskim] forbid this; see K.H.C. 559:74 and M.A. 559:11. Most authorities, however, hold the more lenient opinion; so claim the Mishnah Berurah and [the author of] Torat HaMo’adim (2:5). In practice, though, many communities act strictly. The author of Knesset HaGedolah writes that the Jews of Turkey acted strictly; the author of Shulchan Gavo’ah writes that this was also the custom in Salonika; and Responsa Pe’ulat Tzaddik (3:147) cites this as being the custom in Yemen. The Aruch HaShulchan (559:9) writes, “We have never seen or heard of anyone” making a meal on a postponedTish’a B’Av, or even on one of the minor fasts [that was postponed]. See Piskei Teshuvot 559:9. (Regarding the previous halachah: the author of Yechaveh Da’at, 3:40, rules leniently… even if [the woman] feels no pain [at all].)