for a rainy day 以防不时之需（成语）

英文释义
For a future period of unexpected financial need; for a time when a person has less money that required.

例句

My parents taught me to save some money for a rainy day, so if I lose my job, I can survive for a while until I find a new one.

父母教导我，要存些钱以防不时不需，所以如果我失业了，我有能力维持生活，直到找到新工作。