You should save some money for a rainy day.你应该存些钱以备不时之需。

save some money for a rainy day:存钱以备不时之需     例：I'd like to take a vacation with my wife.However, we have decided to save some money for a rainy day.我想跟太太一起去度假，不过后来我们还是决定存些钱以备将来不时之需。