Tisha B’Av is the saddest day of the year and commemorates the destruction of the First Temple in Jerusalem.
A major fast lasts for 25 hours.
Other major calamities have happened to the Jewish people on Tisha B’av, among them the start of the First Crusade, the expulsion of the Jews from England, France and Spain. Word War I broke out on Tisha B’av and the mass deportation from the Warsaw Ghetto also began on the fast day.