From the usual reference library:
The original date of the celebration in Eastern Christianity was January 6, in connection with Epiphany, and that is still the date of the celebration for the Armenian Apostolic Church and in Armenia, where it is a public holiday. As of 2011, there is a difference of 13 days between the modern Gregorian calendar and the older Julian calendar. Those who continue to use the Julian calendar or its equivalents thus celebrate December 25 and January 6 on what for the majority of the world is January 7 and January 19. For this reason, Ethiopia, Russia, Ukraine, the Republic of Macedonia, and the Republic of Moldova celebrate Christmas on what in the Gregorian calendar is January 7; all the Greek Orthodox Churches celebrate Christmas on December 25.