The 22nd letter of the Greek alphabet, Chi, is represented as an X. You've probably seen the Christian fish symbol, called the ichthyus, sometimes containing the Greek letters ΙΧΘΥΣ. ΙΧΘΥΣ (Ichthys) is an acronym for "Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ", (Iēsous Christos, Theou Yios, Sōtēr), which translates into English as "Jesus Christ, God's Son, Savior". So you see, when people use Xmas, they are simply using a Greek abbreviation for Christ. Xmas = Christmas. :) (http://en.wikipedia.org/wiki/Ichthys)
