In the aftermath of the destruction of the Bet-haMiqdash the Prophets of Israel established that the 9th of Ab will be declared a Day of Fasting.
In Tish'a beAb we don't eat or drink for a whole day, almost twenty five hours.
The fast will take place from Saturday night July 28th until Sunday night July 29th (see tomorrow's HOTD for the times in Great Neck, NY).
The fast of Tish'a beAb should be observed by all those who are in good health.
Yoledet: During the first 30 days after birth, or during 30 days after a miscarriage, a woman is exempted from fasting on the 9th of Ab.
Pregnant and nursing women: Similar to Yom Kippur, pregnant and nursing women, in principle, should observe this fast. This year, however, since the 9th of Ab falls on Saturday and the fast is postponed for Sunday, we proceed with more leniency. Pregnant women, and women who are actually breast-feeding their babies do not need to fast on this Tish'a BeAb. If they can and want to fast, they should fast until Sunday morning or ideally until Chatzot ("Halakhic midday", Sunday July 29th 1:05 PM).
Minors: Boys younger than 13 and girls before 12 are completely exempted from fasting. Unlike Yom Kippur, there is no need for children to fast for a few hours. The reason is that while we do educate our children to fast as part of the teshuba(repentance) process of yom Kippur, we should not educate our children to mourn before they formally have to, because hopefully this will be the last year we mourn for the Bet haMiqdash (Amen!).
Chole She-en Bo Sakana: People with a chronic disease like diabetes or patients under treatment or someone with high fever should not do this fast. Many Rabbis recommend that in these cases, when possible, one should try to fast until the morning or Chatzot.
Elders should consult with their physicians to make sure that the fast will not affect their health. If it will, they are exempted (or forbidden) from fasting.
When one is allowed to eat during Tish'a BeAb for health reasons, one should eat only whatever is necessary for his or her health, and not for pleasure, or in excess
√ WATCH HERE "Crash course on Tish'a beAb" by Rabbi Avraham Goldhar.