Tisha B'av
The Fast of the Ninth of Av

The day primarily commemorates the destruction of the first and second Temples: the first time by the Babylonians in 586 BCE, the second by the Romans in 70 CE.

Many other tragedies of the Jewish people have occurred on this day, most notably the expulsion of the Jews from Spain in 1492.

Summary of information about the day:
Historical background - How we observe the day - Synagogue services