The Julian calendar had been established by Julius Caesar in 46 B.C.
Because it was the Catholic pope who ruled on the adoption of the new calendar, many churches not aligned to the papacy ignored it, such as Protestants and the Eastern Orthodox church. Protestants accepted the new calendar in the early 1700s.
In 1922, the patriarch of Constantinople decided that the Gregorian calendar should be followed for the observance of Christmas, but not for Easter, and this edict was followed by many of the other Orthodox churches.
The majority of Orthodox believers, including the Russian Orthodox Church, Egyptian Coptics, Ukrainian churches, Serbs, Macedonia, and the Mount Athos monks in Greece, celebrate Christmas on January 7th. The churches in Romania, Bulgaria, Cyprus, and Greece mark it on December 25th along with other Christian denominations.
The Armenian Orthodox Church observes Christmas Day on January 6th. This was the original date for Christmas until the 4th century, rather than some Julian/Gregorian adjusted date.