Why do people say Xmas?

Answer 1

Because it is Christian shorthand for Christmas, the X standing for Christ.

Way back when, Greek was the language of Christianity. The New Testament was written in Greek. Greek was and is written using the Greek alphabet, not the alphabet we use. You can still see traces of it in old symbols and phrases still used, like calling God 'the alpha and the omega' meaning 'the beginning and the end' because alpha is the first letter of the Greek alphabet and omega is the last. It's like calling him the A to Z.

Another place you might still find Greek in use in modern western Christianity is the Ichthys or fish symbol. You'll have seen it somewhere, it's the classic fish shape that people sometimes use to show they're Christian. You can look up its history somewhere else, but it's called an Ichthys because that is Greek for fish. Why a fish as a symbol of Christianity? Because 'Ichthys' stands for Iēsous Khristos Theou Huios, Sōtēr = Jesus Christ God's Son, Saviour. The 'ch' in Ichthys stands for Christ, and the Greek letter used to mean 'ch' looks a lot like an X.

So, from ancient times Christians have used the Greek letter that looks like an X as an abbreviation of 'Christ'. For the last few centuries, people have used an actual X instead, particularly if they don't know Greek. So, 'Christians' is abbreviated to 'Xians', 'Christ' is abbreviated to 'X' and 'Christmas' is abbreviated to 'Xmas'. This is useful when people want a shorter way of writing something, and is a reminder of Christianity's roots.