He set a little money aside 'for a rainy day'. [=he saved a little money so that he would have it if he needed it in the future]
It's been 'rainy' all day today. [=rain has been falling all day]
I have some money that I'm saving 'for a rainy day'.
(..) ...I pray the Lord my soul to take. Amen. All right, girls, into bed. Quickly, now. No better place to be on a rainy night. In you go, Lindsey, thattagirl. You've got your bear, that's a girl. No. No talking after lights are out. Good night, children. (..)2006 Silent Hill