keep (something) for a rainy day
-To reserve something, especially money, for use in a time or period of unforeseen difficulty, trouble, or need.
ex]
I know you want to buy a new TV with your Christmas bonus, but you should really keep it for a rainy day.
I keep a portion of my wages in a savings account each month for a rainy day. -The Free Dictionary-
●넷플릭스 드라마 버진리버(Virgin river)에서 배운 표현
(Virgin River- season 4, episode 4)
잭이 멜에게 목걸이를 선물하는 장면
∙Mel: When did you even find time to do this?
∙Jack: I bought it a while back.
Just keep it for a rainy day and I figured last night qualified.