By popular request, the reason that it’s abbreviated X-mas is cause of the symbols Chi-Ro, which is the first two letters of “Christ” (in Greek, I think), and it looks like a “P” superimposed on an “X”. It seems to go back (at least) to the Roman Emperor Constantine, who dreamed of the symbol on the eve of battle. It bore the inscription, “In hoc signo vinces,” which means “By this sign, shall you conquer” (although I’ve seen it translated with slightly different wording).