Orthodox christians celebrate christmas on 7 January. This is because they still follow the old Julian calendar, refered to normally as "The Old Calendar" and also 13 days off. The exception being the Greek Orthodox church which switched over to the Gregorian Calendar (The New Calendar) in order to look like a western country. The Orthodox Church in America (OCA) also follows the new calendar. The state within Orthodox countries for the most part followed the old calendar until the communist revolution and soviet expansion. Afterwards both the Julian calendar and christmas were abolished.
An interesting andecote is that the Soviet government tried to subvert christmas by the creation of a character called Grandfather Frost who was an obvious knock off of Santa Claus. This was intended to destroy the meaning behind christmas without destroying the spirt behind it. Ironically, the Orthodox church did the same thing to Pagan Saturnalia.
Within the Orthodox church there has been some controversy about the old and new calendar switches, in the case such things interest you. Needless to say, people'll fight over anything.