Xmas or X’mas

I didn't know this thing. But when we use a shorthand for Christmas, X'mas is wrong way and Xmas is the right thing. We use the letter 'X' as the 'Christ', we don't need to put an apostrophe between X and m. One of my friend, actually, she is my friend's sister advised me this thing in her reply to my Christmas card. Thank you so much for letting me know.