Why We Mourn Jerusalem's Destruction on Tisha B'Av

Fasting and praying for 25  hours -- many of them sweltering -- isn't anyone's idea of summer vacation. But that's not the only reason Tisha B'Av (the 9th day of the Hebrew month of Av, which begins Saturday night at sundown) is among the least observed days on the Jewish calendar, despite its status of a "major" fast day shared only with Yom Kippur.