Russian Orthodox Church is so orthodox that is still refuses to recognise this new calendar and uses Julian calendar. So Chrismas happens on Jan 7 and will soon move to Jan 8.
That's not specific to Russian Orthodox church, other orthodox churches do it as well (Balkans, Israel, Ethiopia, Egypt etc).
Not all orthodox churches: Greek and Bulgarian churches are on Gregorian calendar (technically on Revised Julian[1] which is aligned with Gregorian until the year 2800).