a rainy day

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 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]

a rainy day

a possible time of need, usually financial need, in the future.

The expression may originate from the days when casual farm labourers needed to save a proportion of their wages ‘for a rainy day’, i.e. for occasions when bad weather might prevent them from working and earning money.

2002 New York Times Book Review The Russian walked out of K.G.B. headquarters with ‘insurance against a rainy day’—the K.G.B.'s file on its secret mole inside the F.B.I.