Actually the Orthodox Church celebrates the Feast of the Nativity on December 25th.
Those on the Old (Julian) Calendar — Russia, Serbia, Georgia, Jerusalem — just happen to have the Julian December 25th coincide with the civil and Gregorian calendars’ January 7th — for now — the drift caused by the Gregorian calendar skipping leap years at the beginning of some centuries will push it to January 8th eventually.
Those on the New “Reformed Julian” Calendar (which coincided with the Gregorian until, should the Lord tarry, the 26th century when it will diverge, becoming more astronomically accurate, again absent intervening calendar reforms) celebrate on December 25th on that calendar, which coincides with December 25th on the civil and Gregorian calendars.
So do you believe Christ was crucified on Friday and raised on Sunday?