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 have occurred on the ninth of Av.

Tisha B'Av means "the ninth (day) of Av." 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 586 B.C.E.; the second by the Romans in 70 C.E.).

Although this holiday is primarily meant to commemorate the destruction of the Temple, it is appropriate to consider on this day the many other tragedies of the Jewish people, many of which occurred on this day, most notably the expulsion of the Jews from Spain in 1492.

Tisha B'Av is the culmination of a three week period of increasing mourning, beginning with the fast of the 17th of Tammuz, which commemorates the first breach in the walls of Jerusalem, before the First Temple was destroyed. During this three week period, weddings and other parties are not permitted, and 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 Shabbat) 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; engaging in sexual relations; and studying Torah. 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.
http://www.jewfaq.org/holidayd.htm

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