The Three Weeks is a period of mourning commemorating the destruction of the first and second Jewish Temples. The Three Weeks start on the seventeenth day of the Jewish month of Tammuz — the fast of Shiva Asar B'Tammuz — and end on the ninth day of the Jewish month of Av — the fast of Tisha B'Av, which occurs exactly three weeks later. Both of these fasts commemorate events surrounding the destruction of the Jewish Temples and the subsequent exile of the Jews from the land of Israel. According to conventional chronology, the destruction of the first Temple, by Nebuchadrezzar II, occurred in 586 BCE, and the second, by the Romans, in 70 CE. Jewish chronology, however, traditionally places the first destruction at about 421 BCE.
