What is Tisha B'Av? Why I Still Fast on Tisha B’Av Tisha B'Av (the ninth day of the Hebrew month of Av) is the culmination of three weeks of mourning - beginning with the 17th of Tammuz - in which the Jewish people commemorate the breach of the walls of Jerusalem during the era of the First Temple, which ended with the Temple's destruction in 587 BCE. It is considered the saddest day in the Jewish calendar, and takes place on the anniversary of the destruction of both Temples in Jerusalem (the second Temple was destroyed in 70 CE). According...