Xmas
This form, which strikes most of us as modern, is actually old, and it is certainly not, as some suggest, a modern plot against Christmas: “Who took Christ out of Christmas?” The OED’s first recorded use of Xmas for Christmas dates back to 1551. But the use of X for Christ is even older. Its first recorded use (in English) dates back to 1021, and it was used by a monk, not by a secularist, transcribing a classical manuscript into Old English. The use of X is simply due to the fact that this is the first letter of Christ in Greek, ΧΡΙΣΤΟΣ, and ΧΡ or just Χ were used from early times onwards as a short form of referring to Christ. (O’Conner 2010: 77)