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.
I'll put the rest in the bank for a rainy day.
See full dictionary entry for rainy
Examples of 'for a rainy day' in a sentence
for a rainy day
But it was a real godsend as my savings were for a rainy day.
The Sun (2008)
In the tenth slot was'Use money on experiences rather than savings for a rainy day '.