This coming Sunday, Dec 27, is Asara beTebet (10th of Tebet) a fast day, which commemorates several tragedies that happened to the Jewish people.
Yesterday we mentioned 1. The translation of the Torah. 2. The death of Ezra haSofer.
The main tragedy that happened in this day was the onset of the siege of Jerusalem by Nebuchadnezzar, the King of Babylonia, and the beginning of the battle that ultimately destroyed Jerusalem and the Bet haMikdash in the year 586 BCE, and sent the Jews into the 70-year Babylonian Exile. The date of the Tenth of Tebet is recorded for us by the prophet Yechezkel, who himself was already in Babylonia as part of the first group of Jews exiled there by Nebuchadnezzar, 11 years earlier than the actual destruction of the Temple in Jerusalem itself.
For more information see: http://www.ou.org/chagim/roshchodesh/tevet/fast.htm