After the 1917 Revolution, Christmas was banned throughout Russia, along with other religious celebrations. It wasn't until 75 years later, after the collapse of the Soviet Union in 1992, that the holiday was openly observed.
Do the Russians celebrate Christmas? Yes they do — thirteen days after the rest of the world does — on January 7th. Why so late? Because the Russian Orthodox Church celebrates Christmas in accordance with the old Julian calendar, which, in this century, is thirteen days out of sync with our Gregorian Calendar.
The Julian calendar (introduced by Julius Caesar in 46 BC) was used in Europe until 1582. The Julian calendar was not so precise (it was 11 minutes and 14 seconds longer than the solar year) so by the year 1580 the calendar had accumulated 10 extra days. Pope Gregory XIII fixed this discrepancy, and introduced what is now known as the Gregorian Calendar.
Not all countries accepted the new calendar immediately. Non-Catholic countries objected to adopting a Catholic invention. England and the United States, for example, did not adopt the Gregorian calendar until 1752 — by which time it was necessary to correct for eleven days.
Russia did not accept the new calendar until 1918, after the Russian Revolution: as a consequence, the anniversary of the "October" Revolution (October 25th) now falls on November 7th.
The Russian Orthodox Church, however, has steadfastly refused to adopt the Gregorian calendar (imposed in Russia by a decree of the Council of People's Commissars) and still uses the old Julian calendar. Thus Christmas is celebrated in Russia not on December 25th, but on January 7th.