Tisha B'Av.

 Tisha B'Av, a day of mourning also known as "The Ninth of Av." It commemorates a number of  calamities that have befallen the Jewish people - all on the ninth day in the Hebrew month of Av. From the destruction of the two Temples in 587 BC and 70 AD respectively, to the expulsion of the Jews from England and Spain in the years 1290 and 1492, it's safe to say that this has been a horrendous date in Jewish history.