Usage note The abbreviation Xmas for Christmas dates from the mid 16th century. The X is the Greek letter chi, the initial in the word Χριστός (Christos) “Christ.” In spite of a long and respectable history, today Xmas is offensive to many, perhaps because of its associations with advertising. It is not used in formal writing.
"Christmas," 1551, X'temmas, wherein the X is an abbreviation for Christ in Christmas, from first letter of Gk. Christos "Christ" (see Christ). The earlier way to abbreviate it was Xp- or Xr-, corresponding to "Chr-," and the form Xres mæsse for "Christmas" appears