Tisha b’Av is the saddest day of the Jewish calendar. On the ninth day of the Hebrew month of Av, Jews remember the tragedies that have marked our history, beginning with the destruction of our Temple in Jerusalem in 586 B.C.E. and continuing through the centuries of expulsions, pogroms and massacres to the catastrophes of our own era. The day is marked by the signs and customs of mourning. From sundown Monday night until nightfall Tuesday evening, Jews engage in a complete fast—abstaining from eating, drinking, bathing, wearing leather shoes, putting on cosmetics and from sexual activity. During the synagogue...