Tisha B'Av, the Fast of the Ninth of Av, is a day of mourning to commemorate the many tragedies that have befallen the Jewish people, many of which coincidentally occurred on the ninth of Av.
Tisha B'Av literally means "the ninth (day) of Av" in Hebrew. It usually occurs during August.
Tisha B'Av primarily commemorates the destruction of the first and second Temples, both of which were destroyed on the ninth of Av (the first by the Babylonians in 423 B.C.E.; the second by the Romans in 69 C.E.).
Although this holiday is primarily meant to commemorate the destruction of the Temple, it is appropriate to consider the many other tragedies of the Jewish people that also occurred on this day. Among them are: the crushing of the Bar-Kokhba Revolt at the hands of the Romans in 133 C.E.; the expulsion of the Jews from England in 1290 C.E.; the expulsion of the Jews from Spain in 1492; and the beginning of World War I in 1914, which by general historical consensus led to World War II and the Holocaust.
Tisha B'Av is the culmination of a three-week period of incrementally increasing mourning, beginning with the fast of the 17th of Tammuz that commemorates the first breach of the walls of Jerusalem, before the First Temple was destroyed. During this three-week period, weddings and other parties are not permitted, and many Jewish people refrain from cutting their hair. From the first to the ninth of Av, it is customary to refrain from eating meat or drinking wine (except on the Sabbath) and from wearing new clothing.
The restrictions on Tisha B'Av are similar to those on Yom Kippur: to refrain from eating and drinking (even water); washing, bathing, shaving or wearing cosmetics; wearing leather shoes; and engaging in sexual relations. Work in the ordinary sense of the word is also restricted. People who are ill need not fast on this day. Many of the traditional mourning practices are observed: people refrain from smiles, laughter and idle conversation, and sit on low stools.
In synagogue, the book of Lamentations is read and mourning prayers are recited. The ark where the Torah is kept is draped in black.