The word Christmas originated as a compound meaning "Christ's Mass". It is derived from the Middle English Christemasse and Old English Cristes mæsse, a phrase first recorded in 1038. "Cristes" is from Greek Christos and "mæsse" is from Latin missa. In Greek, the letter Χ (chi), is the first letter of Christ, and it, or the similar Roman letter, has been used as an abbreviation for Christ since the mid-16th century. Hence, Xmas is often used as an abbreviation for Christmas

Quoted from Wikipedia Web side: http://en.wikipedia.org/wiki/Christmas