Ortodoxa : The difference in Christmas celebrations stretches back to 1582, when Pope Gregory XIII ruled the Catholic Church should follow a new calendar – called the Gregorian calendar, more in sync with the sun than the Julian calendar. The Julian calendar was established by Julius Caesar in 46 B.C.
Because it was the pope who ruled on it, many churches not in sync with the Vatican ignored it, Protestants and Eastern Orthodox among them. Protestants accepted the new calendar in the early 1700s.
In 1922, the patriarch of Constantinople decided to follow the new Gregorian calendar in observance of Christmas, but not for Easter. His lead was followed by many other Orthodox churches.
The only Orthodox churches that observe the Jan. 7 date are the Russian Orthodox Church, the Ukrainian churches, the Serbs and the Mt. Athos monks in Greece, work on weekends and non-working holidays is not overtime work.