Christmas:
The word "Christmas" means "Mass of Christ," later shortened to "Christ Mass." The even shorter form "Xmas" - first used in Europe in the 1500s is derived from the Greek alphabet, in which X is the first letter of Christ's name: Xristos, therefore "X-Mass."

Xmas:
This abbreviation for Christmas is of Greek origin. The word for Christ in Greek is Xristos. During the 16th century, Europeans began using the first initial of Christ's name, "X" in place of the word Christ in Christmas as a shorthand form of the word. Although the early Christians understood that X stood for Christ's name, later Christians who did not understand the Greek language mistook "Xmas" as a sign of disrespect