During a rainy day, season, or period it rains a lot.
例句
They walked along the promenade on a rainy night...他们在一个雨夜沿着海滨道散步。The rainy season in the Andes normally starts in December.安第斯山区的雨季通常在12月份来临。
PHRASE
以备不时之需;未雨绸缪
If you say that you are saving something, especially money, for a rainy day, you mean that you are saving it until a time in the future when you might need it.
例句
I'll put the rest in the bank for a rainy day.我要把余款存入银行以备不时之需。