For westerners living in Georgia, it is perplexing as to why Georgian Orthodox Christians celebrate Christmas on January 7, in common with the Churches of Russia and Serbia. Even more perplexing is that the Orthodox Churches of Greece, Constantinople, Romania, Bulgaria, Antioch, Jerusalem and Alexandria celebrate it on December 25.
Throughout the Orthodox World, the old Julian calender was maintained for Ecclesiastical events until quite recently. The civil authorities in the Slavic Lands (the Russian Empire, Serbia and Bulgaria) maintained the Julian Calendar until the early 20th century; Church authorities continued using the Julian Calendar regardless. Various Patriarchates decided to adopt the Gregorian Calendar during the 20th century but Russia, Serbia and Georgia refused to change. They continue celebrating Christmas on December 25 according to the Julian Calendar, but that falls on January 7 according to the Gregorian calendar used by the civil authorities. The result is that the latter three Patriarchates celebrate Christmas two weeks later than their western and southern co-religionists.
This article by Roman Catholic writer William Tighe examines why the date December 25 in the Julian Calendar was appropriated by the early Church for the date of the Nativity in the first place.
“Many Christians think that Christians celebrate Christ’s birth on December 25th because the church fathers appropriated the date of a pagan festival. Almost no one minds, except for a few groups on the fringes of American Evangelicalism, who seem to think that this makes Christmas itself a pagan festival. But it is perhaps interesting to know that the choice of December 25th is the result of attempts among the earliest Christians to figure out the date of Jesus’ birth based on calendrical calculations that had nothing to do with pagan festivals.
Rather, the pagan festival of the “Birth of the Unconquered Son” instituted by the Roman Emperor Aurelian on 25 December 274, was almost certainly an attempt to create a pagan alternative to a date that was already of some significance to Roman Christians. Thus the “pagan origins of Christmas” is a myth without historical substance.