a rainy day
A time or period of unforeseen difficulty, trouble, or need. I know you want to buy a new TV with your bonus, but you should really save that money for a rainy day. I put aside a portion of my wages each month for a rainy day.
rainy day, a
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]