1
general::
Just prior to, as in On the eve of the conference the main speaker backed out. This expression uses eve, literally "the night before," more loosely. [Late 1700s]
American Heritage Idioms
2
general::
Phrase(s): on the eve of something
Fig. just before something, possibly the evening before something. • John decided to leave school on the eve of his graduation. • The team held a party on the eve of the tournament.
McGrawhill's American Idioms And Phrasal Verbs