If someone wants to “save something for a rainy day”, it means they want to do something, but not right now. They want to save it for the perfect time, for example waiting to do an indoor activity like cleaning for a day when the weather outside is bad. It is also sometimes an excuse to put off for later something you DON`T want to do.

“Do you want to go see this documentary about the Italian Renaissance?”          “Let`s save it for a rainy day…”