Orthodox Christians celebrate Christmas on January 7. Why? Because some Orthodox Christians still follow the Julian calendar for holy dates like Christmas and Easter. And this is the day on which Christ was born according to their calendar. (Never mind that nobody really knows when he was born.)
Of course, the vast majority of Christians follow the Gregorian calendar, which was introduced by Pope Gregory XIII in 1582, ironically, for scientific not religious reasons. Primarily, it was to correct a flaw in the Julian calendar having to do with how much time it assumes elapses between vernal equinoxes.
But this is not the occasion for any further comment on that.
To my Serbian and Russian friends, and to all of my Orthodox readers around the world:
Srecan Bozic, S Rozhdestvom, Happy Christmas!