Tisha B'Av (Hebrew: תשעה באב‎ or ט׳ באב, "the Ninth of Av,") is an annual fast day in Judaism, named for the ninth day (Tisha) of the month of Av in the Hebrew calendar. The fast commemorates the destruction of both the First Temple and Second Temple in Jerusalem, which occurred about 655 years apart, but on the same Hebrew calendar date. Although primarily meant to commemorate the destruction of the Temples, it is also considered appropriate to commemorate other Jewish tragedies that occurred on this day, most notably the expulsion of the Jews from Spain in 1492, one of the concluding events of the Iberian Reconquista.  Accordingly, the day has been called the "saddest day in Jewish history".

Tisha B'Av falls in July or August in the western calendar. When Tisha B'Av falls on the Sabbath (Saturday) observance of Tisha B'Av takes place on Sunday; no outward signs of mourning intrude upon the normal Sabbath. While the day recalls general tragedies which have befallen the Jewish people over the ages, the day focuses on commemoration of five events: the destruction of the two ancient Temples in Jerusalem, the sin of ten of the twelve scouts sent by Moses who spoke disparagingly about the Promised Land, the razing of Jerusalem following the siege of Jerusalem in 70 CE, and the failure of the Bar Kokhba revolt against the Roman Empire.

The fast lasts about 25 hours, beginning at sunset on the eve of Tisha B'Av and ending at nightfall the next day. In addition to the prohibitions against eating or drinking, observant Jews also observe prohibitions against washing or bathing, applying creams or oils, wearing leather shoes, and engaging in marital relations. In addition, mourning customs similar to those applicable to the shiva period immediately following the death of a close relative are traditionally followed for at least part of the day, including sitting on low stools, refraining from work and not greeting others.

The Book of Lamentations is traditionally read, followed by the kinnot, a series of liturgical lamentations.