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. Tisha B’Av is never observed on Shabbat. If the 9th of Av falls on a Saturday, the fast is postponed until the 10th of Av.

-from hebcal.com