The origin of the abbreviation "Xmas" for Christmas from Dappled Things:
The 'X' in question is not, in fact, the usual Latin letter, but rather the Greek letter 'chi.' This is the same chi that you see in church joined to a 'P' -- which is, of course, not a 'P,' but rather the Greek letter 'rho.' The chi, usually together with the rho, is an ancient monogram for Christ, inasmuch as these are the two letters that begin the word Christ -- Messiah -- in Greek. You find it in digs from Christian antiquity, you see it used in mediaeval religious manuscripts, and you find it in the modern 'Xmas.'
I learn something new everyday.