What is the origin of the word Christmas?

Christmas comes from the Old English words Cristes moesse, 'the mass or festival of Christ'. The first celebration took place in Rome about the middle of the fourth century. The exact date of the Nativity is not known, but even in pre-Christian times the period between December 25 and January 6 was considered a special time of year - now known as "The Twelve Days of Christmas". The abbreviation Xmas, thought as sacrilegious by some, is entirely appropriate. The letter X (chi) is the first letter in the Greek word for Christ.