XMAS is also an entirely correct abbreviation for Christmas, with 'X' standing for 'Christ.'

The 'X' comes from the Greek letter Chi, which is the first letter of the Greek word Christós (Greek: Χριστός, translit. Khristós, lit. "anointed, covered in oil"), which became Christ in English.

The suffix -mas is from the Latin-derived Old English word for Mass.

• Wikipedia, but I've heard this from other sources, too, over the years.