What is the X in X-mas?
early 13c., originally especially of the day of a person's death, from Medieval Latin anniversarium, from Latin anniversarius (adj.) "returning annually," from annus (genitive anni) "year" (see annual (adj.)) + versus, past participle of vertere "to turn" (see versus). The adjective came to be used as a noun in Church Latin as anniversaria (dies) in reference to saints' days. An Old English word for "anniversary" (n.) was mynddæg, literally "mind-day."