[Taken from chabad.org]
On the 10th of Tevet of the year 3336 from Creation (425 BCE), the armies of the Babylonian emperor Nebuchadnezzar laid siege to Jerusalem. Thirty months later — on Tammuz 9, 3338 — the city walls were breached, and on Av 9th of that year, the Holy Temple was destroyed. The Jewish people were exiled to Babylonia for 70 years.
Tevet 10 (this year December 17, 2010) is observed as a day of fasting, mourning and repentance. We refrain from food and drink from daybreak to nightfall, and add the Selichot and other special supplements to our prayers. More recently, Tevet 10 was chosen to also serve as a “general kaddish day” for the victims of the Holocaust, many of whom the day of their martyrdom is unknown.