I'm going to save it for a rainy day.
 →  만약을 위해 돈을 모을라고.

[예문]
save(something) for a rainy day:만약을 위해 돈을 모으다

A:What are you going to buy with your bonus?
  보너스로 뭘살거니?

B:Nothing. I'm going to save it for a rainy day.
  아무것도.만약을 위해 모을라고.