“The Tenth of Tevet marks 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 Temple of Solomon, and sent the Jews into the 70-year Babylonian Exile.
However, there are other commemorative days that fall immediately before the Tenth of Tevet and their memory has been silently incorporated in the fast day of the Tenth of Tevet as well.
On the eighth of Tevet, King Ptolemy of Egypt forced 70 Jewish scholars to gather and translate the Hebrew Bible into Greek… This translation… aided the attempt to reform Judaism in the image of Greek values and lifestyle…
The ninth day of Tevet is… the day of the death of Ezra the Scribe. [He] is responsible for the survival of Judaism and the Jews till this very day.
Since fasting on the eighth, ninth, and 10th days of Tevet consecutively would be unreasonable, the events of the eighth and ninth were subsumed into the fast day of the Tenth of Tevet.”