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.
2002New 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.