X is the Greek letter chi, which is the first letter of the word Christ. Thus Xmas is shorthand for Christmas, taking only about one-sixth as long to write. If you do your cards by hand, it is possible to save as much as seventy-five or eighty minutes a year.
It is tempting to say that what you do with this time that you save is your own business. Briefly stated, however, the Christian position is that there's no such thing as your own business.