Kislev 4

In 346 BCE, a delegation of Babylonian Jews arrived in Jerusalem to ask the prophet Zechariah if the fast of Tisha B'Av should be discontinued (Zechariah ch. 7). Tisha B'Av is a commemoration of the destruction of the Temple, and at the time, the Second Temple had just been constructed. The answer, as recorded in the Talmud, is that if Israel remains under foreign control, then the fast remains -- even if the Temple is built. But if the Temple is built and Israel is self-governed, then the fast turns into a day of celebration. In this case, since the Second Temple was eventually destroyed (also on Tisha B'Av, 420 years later), it is commemorated till today as a Jewish national day of mourning.