Following the election of Mikhail Gorbachev, and the introduction of his reform policies in 1986, the Soviet Union began it's slow breakup. The Berlin wall came down on November 9th, 1989; and the USSR officially ceased existence as a political entity on December 25th, 1991, with the resignation of it's last president, the same Mikhail Gorbachev.
The soviet flag was lowered for the last time on December 31st, 1991.
Another reason to celebrate the holiday season. The end of the only constitutionally socialist state. I can't think of a better reason, myself.