Tisha B'Av, the ninth of the month of Av, is the day when we remember the destruction of both Temples that once stood in Jerusalem as well as a number of other tragedies that have befallen the Jewish people over the course of history.
Ancient rabbinic sages held that the ninth of Av was preordained to be a day of tragedy because of a story in the Talmud (Numbers 13-14) that took place that day: The spies Moses had sent to scout the Promised Land came back with a frightened report, and the people wept at the prospect of entering a land full of giants. God declared to them, "You wept without cause; I will therefore make this an eternal day of mourning for you."
The Talmudic tractate Taanit states that God then decreed that on the ninth of Av, the Temple would be destroyed and Israel would go into exile.
The following rituals are traditionally observed before and during Tisha B'Av:
- A three-week mourning period preceding Tisha B'Av begins on the 17th day of the month of Tammuz.
- Nine days prior to Tisha B'Av, a new period of more intense mourning begins. Traditional Jews do not eat meat, cut their hair, or wash their clothes. All these actions are considered signs of joy or luxury inappropriate for this time of mourning. The Shabbat immediately preceding the holiday is called Shabbat Hazon.
- Tisha B'Av is a full fast day, so the last meal, called a "seudah ha-mafseket," must be eaten before sunset prior to the ninth of Av. The meal often is comprised of round foods like eggs or lentils, which symbolize mourning in Jewish tradition because they evoke the cycle of life. Some people eat an egg or bread sprinkled with ashes, and some sit on the ground during the meal.
- Uniquely on Tisha B'Av, Torah study, meant to be joyful, is not permitted.
- The traditional Torah reading erev Tisha B'Av is Eicha, or Lamentations. During the day, it is Deuteronomy 4:25-40 and the Haftarah is Jeremiah 8:13-9:23.
- The meal ending the fast traditionally omits meat and wine, in acknowledgment of the fact that the burning of the Temple continued until the next day.
The Shabbat immediately following Tisha B'Av is called Shabbat Nahamu (Shabbat of comfort) because the haftarah begins with the words "nahamu nahamu ami" ("comfort, comfort my people"). This begins a period of consolation and comfort leading up to Rosh Hashanah, the Jewish New Year.
This background on Tisha B'Av was compiled from MyJewishLearning.com. Click here  for more information about Tisha B'Av.