From Glossary of Meteorology
(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.
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