X-MAS is how my father used to write “Xmas” on my present when I was a kid, living in a trailer park. So soooorry if that’s how I prefer to write it to honor my childhood.
The X in X-mas is from the Greekletter Chi, which is the first letter of Χριστός, Christ in Greek.
I’d always been told it was a ‘criss-cross’ rather than an X, hence ‘criss-mas’. The philologist in me likes this explanation better though.