save for a rainy day, to

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 put something aside for a future time of need. To keep something for future use is a very old concept indeed; to call hard times a “rainy day” dates from the sixteenth century. Nicholas Breton used it in 1582 (Works): “Wise men say keepe somewhat till a rainy day.” Alternative locutions include to lay up for a rainy day (John Clarke, 1639), laying by against a rainy day (Samuel Pepys, 1666), and putting something by for a rainy day. Ring Lardner (Anniversary, 1926) used it ironically: “Louis was saving for a rainy day, and his wife had long ago given up praying for rain.” See also salt away.

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.