I will not come on a rainy day.

我不会在雨天来。

Oh, great! Today is a rainy day.

哦，太棒了！今天是个雨天。

Keep an umbrella for a rainy day.

下雨天要带把伞。

What happens when it's a rainy day?

如果是下雨天会怎么样？

A rainy day is cold in this season.

这个季节雨天很冷。