on the eve of
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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.