late 14c., "the evening star," from Old French vespre "evening, nightfall" (12c., Modern French vêpre), from Latin vesper (masc.), vespera (fem.) "evening star, evening, west," related to Greek hesperos, and ultimately from PIE root *wes-pero- "evening, night" (source also of Old Church Slavonic večeru, Polish wieczór, Russian vecherŭ, Lithuanian vakaras, Welsh ucher, Old Irish fescor "evening"), perhaps an enlarged form of root *we- "down" (source of Sanskrit avah "down, downward"), thus literally "direction in which the sun sets." 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).