Tisha B'Av is a fast day that commemorates two of the saddest events in Jewish history that both occurred on the ninth of Av - the destruction in 586 BCE of the First Temple, originally built by King Solomon, and destruction of the Second Temple in 70 CE. Other calamities throughout Jewish history are said to have taken place on Tisha B'Av, including King Edward I's edict compelling the Jews to leave England (1290) and the Jewish expulsion from Spain in 1492.
(Adapted from Wikipedia)