To save something for a rainy day 意為「存錢以備不時之需；有備無患；未雨綢繆」，與我們常說的「天晴防下雨」概念不謀而合。這個慣用語中的 something 通常是指金錢，而動詞 save 可用 keep、put away、put aside 或 set aside 等同義詞來替換。
這個成語可以追溯到約 1561 年，首次出現在 Antonio Francesco Grazzini 所寫的義大利文戲劇 La Spiritata 中。英國作家、詩人暨劇作家約翰‧利利 (John Lyly) 於 1580 年將該戲劇改編為英文版並改名為 The Bugbears。
例句：
- Luckily my mother had saved some money for a rainy day. (幸好我母親未雨綢繆早有準備)
- I keep a portion of my wages each month for a rainy day. (我未雨綢繆每月都儲蓄一部份薪水以備不時之需)
- He put away some money for a rainy day. (他存了一些錢以備不時之需)
- I know you want to buy a new TV with your productivity bonus, but you should really put/set that money aside for a rainy day. (我知道你想用生產獎金買一台新電視，但你其實應該把那筆錢存起來以備不時之需)