The idiomatic expression "save for a rainy day" means to set aside money or resources for future use, particularly in case of unexpected expenses or emergencies. Full definition
In most cases however, people tend to focus on simply saving for a rainy day in order to create nest eggs and ensure that their future is trouble free. (policybazaar.com)
When debt prevents saving for a rainy day fund, it may also interfere with your ability to save for retirement. (wisebread.com)
So, if you don't have any savings, now is the time to start saving for that rainy day. (jessifearon.com)