Some people object to writing Xmas, rather than Christmas, saying that it is taking the Christ out of Christmas, and a sign of increasing commercialism. In fact, it has been around for a long time - the first OED citation (spelled X'temass) is dated 1551.
The letter X is a long-standing abbreviation for Christ - it appears in the Anglo-Saxon Chronicles of a thousand years ago (in the phrase 'On Xp̄es mæsse uhtan'). X, called chi in the Greek alphabet, is the first letter in the Greek spelling of Christ. Xp (chi and rho) are the first two letters and they were often combined to form a symbol that represented Christ (see the symbol on this Wikipedia page).
A Merry Christmas everyone.