Siman 550

Everyone is obligated to fast these four fasts, and it is forbidden to break the fence. And even those with eye ailments, if the fast will not damage his eyes. And so too anyone with an ache, if they are not of the aches for which fasting is difficult for them, such as aches of the heart and the head—they shall fast. And pregnant and nursing women—they shall not fast on all of the three fasts, except for Tisha B'Av. However, healthy women have the custom to be stringent on the three fasts. And even though regarding a fast for rains it is forbidden for them to be stringent, as I will write in siman 575, nevertheless regarding fasts established for all of Israel—they are permitted to be stringent. However, when they suffer greatly—it is forbidden for them to be stringent, even though there would be no danger in this for her or for the fetus (and see Magen Avraham subsection 2). And so too one who is weak in his body—he should not be stringent upon himself to fast, except for Tisha B'Av.

And these three fasts—it is permitted to wash, to anoint, to wear leather shoes, and to have marital relations. And one does not need to begin the fast while it is still day, for nowadays there is no public fast except for Tisha B'Av. And all these fasts, if they fall on Shabbat—we postpone them until after Shabbat. And why do we not move them earlier to before Shabbat, as we do with the Fast of Esther? Because we do not move a punishment earlier. However, according to our fixed calendar, it is impossible for the Tenth of Tevet to fall on Shabbat, and only on Friday can the Tenth of Tevet fall, and then we fast on it and complete it, and we read "Vayechal" in the morning and at Mincha. And even though the members of the Ma'amad would not read at Mincha on Friday out of respect for Shabbat, as it says in Taanit (26a), that was because they would not fast on Friday, as is explained there, which is not the case with a public fast. Furthermore: there it was every week, but here—it is an occurrence of many years (see Magen Avraham, subsection 6, and in my humble opinion it is as I have written). And we do not say confessions at Mincha, nor "Avinu Malkeinu," but we do say "Anenu." And if there is a chuppah on the day of the fast on Friday—it is the custom to pray Mincha and to read "Vayechal," and afterward they perform the chuppah. And the groom and bride shall not drink from the cup, for they must fast until the evening, and children shall drink the cup. And all the more so when a fast falls during the seven days of feasting, that they are obligated to fast. But for a Yahrzeit one should not fast during the seven days of feasting (see Taz). And know that Tisha B'Av, the Seventeenth of Tammuz, and the Fast of Gedaliah—it is impossible for them to fall on Friday according to our fixed calendar.

From the perspective of the law, there is no prohibition of washing in hot water during any of the three fasts; nevertheless, the custom is to forbid it, except on the eve of Shabbat when one washes in hot water for the sake of the honor of Shabbat (Bach). However, regarding cold water, there is no custom at all, and one may wash in cold water. Similarly, to sweat in a bathhouse is also permitted, as this is not a pleasure but rather the removal of the discomfort of sweat.

Our teacher, the Beit Yosef, wrote in section 4 that on the Shabbat before the fast—the sheliach tzibur announces the fast, except for Tisha B'Av, and Yom Kippur, and Ta'anit Esther; and your mnemonic is "akaf alav pihu": A = Av, K = Kippur, P = Purim. And the reason is because these three fasts are not dependent on the will of the community. But the seventeenth of Tammuz, and Tzom Gedaliah, and the tenth of Tevet—they said in the Gemara there: If they wish, they fast, etc. Therefore, they announce that they wish to. And therefore, according to that which we hold—that now it is not dependent on will, for we have already accepted the fasts upon ourselves. And therefore, our teacher, the Rema, wrote that the custom of the Ashkenazim is not to announce any of them. Thus far his words. Because now it is not dependent on will (the Gra).