save for a rainy day

Fig. to reserve something--usually money--for some future need. I've saved a little money for a rainy day. Keep some extra allowance for a rainy day.

to keep something, esp. money, for a time in the future when it might be needed It looks like people may be saving a little more for a rainy day.

to keep an amount of money for a time in the future when it might be needed She has a couple of thousand pounds kept aside which she's saving for a rainy day.

Provide contingency funds for when times are tough. There's no clear answer to when this expression began (some have traced it back to the 16th century), but it's clear that a “rainy day” is the symbol of gloom. The wise course, therefore, is to sock away funds to tide you over when times are tough.