What is the X in X-mas?
c.1500, "to light up, shine on," a back-formation from illumination, or else from Latin illuminatus, past participle of illuminare (see illumination). Earlier was enlumyen (late 14c.) "decorate written material with gold, silver, bright colors," from Old French enluminer, from Late Latin inluminare; also illumine (late 14c.). Related: Illuminated; illuminating.