Tisha B'Av

Tisha B'Av means the ninth day of [the month of] Av, which is a month in the lunar calendar used for purposes of Jewish holidays, etc.

This is a fast day, that commemorates two of the saddest days in Jewish history -- the destruction of the first Temple originially built by King Solomon, (see Solomon's Temple), and the destruction of the Second Temple. Those two events occurred about 556 years apart, but both in the same month, Av, and both on the ninth day.