A time of need or trouble, as in We knew a rainy day would come sooner or later. This idiom is often used in the context of save for a rainy day, which means to put something aside for a future time of need. [Late 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.