- Peninei Halakha
Children who have yet to reach the age at which they are obligated in the mitzvot are exempt from the rabbinic fasts. Additionally, the Sages did not require us to train our children to fast for a few hours; they did so only in regard to Yom Kippur, which is mandated by Torah law. Nonetheless, many have a custom to train their children to fast for a few hours, each child according to his ability. Despite this, children should not fast for the entire day (Responsa Rema Mi-Fano §111; see Kaf Ha-ĥayim 554:23). When feeding children on a fast day, one should give them only simple foods, in order to teach them to mourn with the congregation (mb 550:5).
Brides and grooms must fast on the minor fast days. Even though they have a mitzva to rejoice for seven days after their wedding, and therefore they may not accept upon themselves a private fast, nonetheless, they must observe public fasts, because public mourning overrides private joy. Moreover, brides and grooms have a special mitzva to remember the destruction of the Temple, as it says in Tehilim (137:6): “If I do not keep Jerusalem in memory even at my happiest hour” (Ritva, bhl 549:1. Many authorities are lenient regarding Ta’anit Esther; see below ch. 14 n. 12).
The main participants in a brit mila – the father, the sandak, and the mohel – also must fast. The same is true of a father who performs a pidyon ha-ben on a fast day; he, too, may not eat. Instead, it is customary to perform the brit or the pidyon toward the end of the fast and conduct the festive meal after tzeit ha-kokhavim.
Soldiers who are engaged in a defensive operation that is liable to be compromised if they fast should eat and drink as they normally do, so that they can carry out their mission properly. However, soldiers who are merely training must fast.
 According to the Vilna Gaon (end of §686), the main participants in a brit and a bridegroom on the day of his wedding need not observe the minor fasts. Most poskim, however, maintain that they must fast. When the fast is postponed, however, the celebrants may eat after the earliest time for praying Minĥa (half an hour after midday), even on Tisha Be-Av. So states sa 559:9. And even though some authorities rule stringently on the matter, as Kaf Ha-ĥayim 559:74 explains, the majority rules leniently. mb and Torat Ha-mo’adim 2:5-6 concur. In practice, however, ahs 559:9 states:
Nevertheless, we have not seen or heard of anyone who does this, especially concerning us, who eat most of our festive meals at night. This is true of all fasts, not only of Tisha Be-Av. And even if a fast is postponed, we do not partake of a festive meal until nighttime, whether it is for a brit mila or a pidyon ha-ben. This is the common practice, and one should not change it.
See below ch. 10 n. 28. If the postponed fast falls out during the seven days of celebration following a wedding, the poskim debate whether the bride and groom must complete the fast. Many authorities are lenient, including the Vilna Gaon. Thus, those who desire may be lenient and fast until the earliest time for Minĥa, and then eat. On Ta’anit Esther, a more lenient fast, one may certainly be lenient (sht 686:16).