Tisha B'Av is a fast day that commemorates the destruction of the two Temples. It is named after its date on the Jewish calendar - the 9th (Tisha) day of the Hebrew month of Av. The 9th of Av usually coincides with late July or mid-August and this year Tisha B'Av falls on July 29th. Leading up to Tisha B'Av is a period of mourning known as The Three Weeks.
Tisha B'Av also happens to be a date on which a number of other tragedies have befallen the Jewish people. For instance, King Edward I of England expelled the Jews from England on this date in 1290 C.E. You can learn more about Tisha B'Av calamities in: Tragedies that Have Occurred on Tisha B'Av.
Image: Roman triumphal procession after sacking the Temple. (Location: The Arch of Titus in Rome)