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There is no simple answer to this question.
Although most Eastern Orthodox churches celebrate Christmas on Dec. 25, they use a different calendar than the Gregorian calendar that is in common use. When viewed from the perspective of the common calendar, Eastern Orthodox Christmas falls on Jan. 7.
Because the New Testament Gospels do not provide a date for the birth of Jesus, it fell to early church leaders to determine a date for what would become Christmas. The Religion Facts website states that as early as 273 A.D., many Christians were observing Christmas on Dec. 25. Pope Julius I made the date official in 320 A.D.
After the split between the Eastern and Western branches of Christianity, many Eastern churches focused on the celebration of Christ's baptism, which was fixed on the Epiphany, Jan. 6. For a time, some of those churches also celebrated the birth of Christ on the same day, but by the 5th Century most eastern churches adopted Dec. 25 as Christmas Day.
Some Eastern Orthodox churches still use the Julian calendar, dating from the time of Julius Caesar, to regulate their feasts and festivals. Because the Julian calendar is less precise at measuring the Earth's journey around the Sun, most of the Western world uses the more accurate calendar proposed by Pope Gregory XIII in 1582.
There is a 13 day difference between the Julian and Gregorian calendars. So, when Eastern Orthodox Christians celebrate Christmas on Dec. 25 by the "Julian," the celebration occurs on Jan. 7 on the calendar that is in ordinary use in the Western world.