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 have occurred on the ninth of Av.
Tisha B'Av means "the ninth (day) of Av." It occurs in July or 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 and from England in 1290.