Although it has been fairly widely debunked by now, the notion that using “Xmas” for “Christmas” is somehow a slight to Christ or Christianity is still fairly widespread. In fact, “X” Wikipedia’s explanation of the abbreviation “X” for “Christ” is as good as any:
The use of “Χ,” derived from Chi, the Greek alphabet initial, as an abbreviation for Christ (most commonly in the abbreviation “Χmas“) is often misinterpreted as a modern secularization of the term. Thus understood, the centuries-old English word Χmas, is actually a shortened form of CHmas, which is, itself, a shortened form for Christmas. Christians are sometimes referred to as “Xians,” with the ‘X’ replacing ‘Christ.
Using “Xmas” can lead to obvious problems, however. If you intend the term in its original sense, you’re liable to draw a lot of seething looks from folks who will reflexively assume you’re a soulless atheist. Yet, if you do intend the term as an affront, you run up against the historical origins and meaning of the abbreviation and wind up potentially drawing more attention to the Son of God. Most probably, little good can come from using the term.
Thus, for the general public, I suggest “Christmas” be left unabbreviated. For atheists and staunch secularists seeking to purge the religious context from Christmas, I suggest getting over yourselves.