Xmas is a short way of writing Christmas. You see it on shop signs, on price labels ("Xmas sale"), on cards, in headlines, and in text messages — anywhere space is tight. It is written much more than it is said; most people read it aloud as "Christmas". Some people do say /ˈeksməs/ out loud, but that sounds joking or a bit cheap, and it is safer for a student not to say it that way. The X is not the English letter X used as a cross-out or an unknown thing. It is the Greek letter chi, the first letter of Χριστός (Christos), so X has stood for "Christ" in church writing for about a thousand years. Because most people today do not know this, some Christians dislike Xmas and feel it "takes Christ out of Christmas" — you will meet this opinion, and it is worth knowing even though the history goes the other way. Register: fine on a poster, a sign, a shop window, a quick note. Not fine in an essay, a formal letter, or anything careful — there you write Christmas in full. Xmas takes the same words around it as Christmas: Xmas Day, Xmas Eve, Xmas tree, Xmas party. There is no apostrophe (X'mas is common on signs in Taiwan and Japan, but English writers do not use it).