Tish B'Av, which translates as the ninth of Av, ends a three week period of semi-mourning starting on the fast of Tammuz, three weeks previous. The ninth of Av, however, is the peak of this mourning period, where tradition prevents us from shaving, eating, and entertainment.
The reasons for this distress are commemorations of events that have happened in the lives of Jews, such as:
The Destruction of the First Temple
The Destruction of the Second Temple
Beginning of the Jewish Exile from Palestine
Various other events
all of which happened on the ninth of Av.
