While today is (Gregorian) New Year's Day, on the Jewish calendar it is a fast day.
This fast commemorates the beginning of the end of the First Temple.
By Dr. Guy Miron for MyJewishLearning.com
The fast day of the 10th of the Hebrew month of Tevet symbolizes the first of a series of events which led to the destruction of the First Temple: the beginning of the siege of the Babylonians on Jerusalem, the capital city of Judea, as the Book of Kings relates:
"Zedekiah rebelled against the king of Babylon.
And in the ninth year of his reign, on the 10th day of the 10th month Nebuchadnezzar moved against Jerusalem with his whole army. He besieged it; and they built towers against it all around. The city continued in a state of siege until the 11th year of King Zedekiah" (Kings II, 25 verse 1-2).
The prophet Yeheskel [Ezekiel] was instructed by God to turn this day into a day of memory:
"O mortal, record this date, this exact day; for this very day the king of Babylon has laid siege to Jerusalem" (Yeheskel 24, verse 2).