The ninth day of the Hebrew month of Av, known as Tisha B'Av, is the saddest day on the Jewish calendar.
The 25-hour fast was initially instituted to commemorate the destruction of the two Holy Temples in ancient Jerusalem and other associated tragedies of that era.
But over the course of Jewish history, the day has accumulated a long list of late summer tragedies, including the beginning of the First Crusade in 1096; the expulsion of the Jews from England in 1290, France in 1306, and Spain in 1492; and the start of World War I in 1914.
Even in times of relative peace, Tisha B'Av is used as a way to express and contain communal sorrow.