c. 1200, eve "evening," especially the time between sunset and darkness, from Old English æfen, with loss of terminal -n (which, though forming part of the stem, perhaps was mistaken for an inflection), from Proto-Germanic *æbando- (source also of Old Saxon aband, Old Frisian ewnd, Dutch avond, Old High German aband, German Abend, Old Norse aptann, Danish aften), which is of uncertain origin. Now superseded in its original sense by evening.
Specific meaning "day before a saint's day or festival" is from late 13c. Transferred sense of "the moment right before any event, etc." is by 1780. Even (n.), evening keep the original form.