As a child, I remember being shocked to see the word Xmas representing Christmas. I don't ever remember being told, but always felt like the use of the X was crossing out Jesus. It is only in later years that I have been informed as to the reason for this:
The X is representative of the Greek first letter of the name Christ.
The first letter of the Greek word Christos is transliterated into our alphabet as an X. That X has come through church history to be a shorthand symbol for the name of Christ.