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 leading contender for valedictorian of his high school, is a 208-month-old boy with disturbing interests in etymology, vexillology, geography, and law. Adam would like to one day visit Tajikistan and just won an essay contest on linguistics!
The Etymology Nerd