From AMS Glossary
(Redirected from Key day)
(Also called key day.) One of several days on which the weather is supposed (according to folklore) to provide the key for the weather of a subsequent period.
The best-known of these days are in the United States, Groundhog Day (2 February); in England, St. Swithin's Day (15 July). Similar beliefs are associated with other days, mostly saints' days, in many countries.