If you ever refer to an eve as the day before an event, such as "Christmas Eve", etymologically speaking you are grammatically incorrect. Eve dates all the way back to Proto-Germanic (but not PIE? What a slacker) and the word aebando, which basically was a synonym of even, though that came from a slightly different root. As Proto-Germanic became Old English, aebando became aefen. This then became aefe because of rampant misspellings, and eve, since ae sounds like e, and f sounds like v in many cases. This changed definition from "even", because like the word equinox, it was meant to describe a period evenly split between day and night. The whole "evening" definition was later modified to mean "evening or a day before an event", so don't worry, because today you are grammatically correct. Fun fact: the name Eve has nothing to do with this Germanic nonsense and came from no-nonsense Hebrew, literally meaning "a living being".
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd