What is the X in X-mas?
late 14c., "the evening star," from Old French vespre, from Latin vesper (masc.), vespera (fem.) "evening star, evening, west," related to Greek hesperos, and ultimately from PIE *wespero- (cf. Old Church Slavonic večeru, Lithuanian vakaras, Welsh ucher, Old Irish fescor "evening"), from root *we- "down" (cf. Sanskrit avah "down, downward"). Meaning "evening" is attested from c.1600.
Vespers "sixth canonical hour" is attested from 1610s, from plural of Latin vespera "evening;" the native name was evensong (Old English æfen-sang).