Five tragedies befell the Jewish people on Shiva Asar B’Tamuz (seventeenth of Tamuz). A- The first Luchos were broken when Moshe Rabbeinu descended from Har Sinai. B- The Karban Tamid (two daily sacrifices), were suspended during the time of the First Beis Hamikdash. C- The enemy penetrated the walls of Yerushalayim prior to the destruction of the Second Beis Hamikdash. D- The wicked Apostumos burned the Torah. E- An idol was erected in the Beis Hamikdash. (See Taanis 26b, Rambam Taanis 5:2 and Halachos of the Three Weeks page 1) This day is commemorated by refraining from eating and drinking from sunrise to nightfall.
The purpose of the fast day is to stir our hearts in order that we utilize the paths of teshuva, repentance, and to serve as a reminder to us of our own actions and those of our forefathers. These iniquities are responsible for the terrible events which befell our people. Remembering these tragedies should lead us to examine our conduct and return to Hashem. One should not mistakenly, assume that abstinence from food and drink alone fulfills the requirement and purpose of the fast day. Rather, the true purpose of a fast day must be evidenced in one’s improved deeds. (Mishnah Berurah 549:1)
This year the fast begins on Sunday (July 24) at 4:33 a.m. and ends at 9:06 p.m. (Eastern Standard Time). Even though the fast begins from dawn, sometimes the prohibition against eating begins from the previous evening. For example, if, before the arrival of dawn, a person decides not to eat any more until the fast begins, he is seen as having accepted the fast upon himself, and it is now forbidden for him to eat. Therefore, if a person goes to bed in anticipation of the fast and then rises before dawn, it is forbidden for him to eat, for he diverted his thoughts from eating. If one wishes to wake up before dawn and eat or drink, he should stipulate before going to sleep that he intends to wake up early to eat or drink and that the fast should not begin until dawn. The Shulchan Aruch (564) rules that if one did not make this stipulation he may not eat or drink. However, according the Rama he may still drink, even without making a stipulation.
Both men and women must fast on Shiva Asar B’Tamuz. (Shulchan Aruch 550:1)
The Shulchan Aruch (554:5) writes that a pregnant woman need not fast on Shiva Asar B’Tamuz. The Rama (550:1), however, writes that if the pregnant or nursing woman is feeling good the custom is to fast. If she feels week she need not fast. Many poskim explain that the accepted practice today, even among Ashkenazic women, is for pregnant and nursing women to refrain from observing the minor fasts. And even a woman who wishes to be stringent and to observe the fast – if she experiences difficulty during her fast, or if the fast causes her to have less milk, thus causing discomfort to the baby, it is best that she refrain from fasting. (Orchos Chaim Spinka 550:4 and Siddur Yaavetz) For normative halacha, a rav should be consulted.
Nonetheless, if the woman isn’t fasting she shouldn’t eat lavish meals with meat and wine, but rather only what is necessary. (Shulchan Aruch ibid. and Mishnah Berurah 550:5)
The pregnant or nursing woman who does not fast need not make up the fast on a different day. (Yechava Daas 1:35)
One who is sick, even if there is no danger of dying is exempt from fasting and shouldn’t fast. Especially this year that the taanis was postponed (nidcha) from Shabbos to Sunday. (See Netai Gavriel Bein Hamitzarim 5:9) Before one decides to break his fast he must first consult with a rav. If one is sick and does not fast he need not make up his fast on a different day. (Yechava Daas ibid. and Mishnah Berurah Biur Halacha 550)
One need not train his children to fast, even at the age of twelve for boys or eleven for girls. Once they have reached the age of chinuch they shouldn’t eat lavish meals, but rather only what is necessary. Harav Shlomo Zalman Auerbach zt”l and ybc”l Harav Neventzhal shlit”a write that although many boys have the custom to fast three fasts before they become bar-mitzvah, this custom has no source. (Mishnah Berurah 550:5, Halichos Shlomo vol. 3 page 398-399 and B’Yitzchak Yikare on Mishnah Berurah)
The Mishnah Berurah (567:11) maintains that only if one is in pain may one rinse one’s mouth on Shiva Asar B’Tamuz and in such a case one should bend one’s head downward so one doesn’t swallow any water.
If one has bad breath and it causes him discomfort or embarrassment, he may use mouthwash on Shiva Asar B’Tamuz. He may also brush his teeth with toothpaste but not with water. (Beer Moshe 8:94, Minchas Yitzchak 4:109)
If one needs to take medicine on Shiva Asar B’Tamuz he may take them without water. If he can not swallow the pills without a little water, he may swallow a very small amount of water (just enough to get the pills down). (Opinion of Rav Debreczin zt”l cited in Nitev Gavriel Bein Hatzomos page 54)
If one ate by mistake on a fast day he should nevertheless continue fasting afterwards. (Shulchan Aruch and Mishanh Berurah 568:1) The Mishna Brurah (568:3) says that even if one ate on a taanis, you can still say aneinu during the shemoneh esrei. Harav Wosner zt”l (Shevet Halevi 5:60) explains that this is only true for someone who began the fast and ate accidentally. However, one who is exempt from fasting altogether does not recite aneinu.
Many have the custom to recite Avinu Malkeinu on public fast days. One can even say Avinu Malkeinu when davening without a minyan. (Ishei Yisroel 45:45)