WordNet (r) 3.0 (2006):

rainy day
    n 1: a (future) time of financial need; "I am saving for a rainy
         day"

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "rainy day":
   bad times, bust, cat-and-dog weather, depression, dirty weather,
   economic stagnation, evil day, hard times, heavy weather, monsoon,
   predominance of Aquarius, raininess, rains, rainy season,
   rainy weather, recession, sad times, slump, spell of rain,
   spring rains, stormy weather, wet, wet season, wet weather