The Question:

I am wondering what the X in Xmas means? Why is Christmas abbreviated this way?

The Answer:

Contrary to popular belief, the X is not a euphemism to make the holiday seem culturally neutral. The Greek word for Christ is Χριστος. The first letter, a chi, looks like the English X. X has accordingly been used as an abbreviation for Christ for centuries. Xmas, like Christmas, thus literally means "Christ's mass."

—The Fact Monster