The date of the “Orthodox Christmas” is not January 7.
It is December 25, but, in some cases, December 25 in the old Julian calendar (which was official in the west between 45 BC and AD 1582).
The Julian December 25 is (until AD 2100) January 7, ie today, in the new Gregorian calendar (1582-).
The autocephalous Orthodox Churches of Jerusalem, Russia, Serbia and Georgia use the old Julian calendar for their fixed dates. So do the minority Greek Old Calendarists. The Ukrainian and Macedonian claims to autocephaly are not properly recognised, but their churches use the Julian calendar.
The autocephalous Orthodox Churches of Constantinople, Alexandria, Antioch, Romania, Bulgaria, Cyprus, Greece, Poland, Albania, and of the Czech and Slovak Lands, and the autocephalous Orthodox Church in America follow the Gregorian calendar. Some OCA parishes use the Julian calendar. So it is Christmas Day in Serbia, but not in Bulgaria.
Despite the autonomy implied in the word autocephaly, there is a ranking for these churches, which I’ve shown here. Constantinople is at the head.
The most commonly used liturgy is the one written by St John Chrysostom (AD 347-407).
Writing this was supposed to be a prelude to an elegant summary of all the complexities of the old Roman, Julian and Gregorian calendars, but that will have to be for another time.
Cathedral Square, Moscow Kremlin, 1956, photograph by David Douglas Duncan