What is the X in X-mas?
late 14c., from Old French equinoce (12c.) or directly from Medieval Latin equinoxium "equality of night (and day)," from Latin aequinoctium "the equinoxes," from aequus "equal" (see equal (adj.)) + nox (genitive noctis) "night" (see night). The Old English translation was efnniht. Related: Equinoctial.