save, keep, etc. it for a rainy day
save (something) for a rainy day
To reserve something, especially money, for use in a time or period of unforeseen difficulty, trouble, or need. I know you want to buy a new TV with your Christmas bonus, but you should really save it for a rainy day. I save a portion of my wages each month for a rainy day.
save, keep, etc. it for a ˌrainy ˈday(informal) save money or things for a time in the future when you might need them: ‘Don’t spend it all at once,’ his aunt said. ‘Save some of it for a rainy day.’
See also: rainy