Wikipedia
Surströmming (, Swedish for "sour herring") is fermented Baltic Sea herring that has been a staple of traditional northern Swedish cuisine since at least the 16th century.
Just enough salt is used to prevent the raw fish from rotting (chemical decomposition). A fermentation process of at least six months gives the lightly-salted fish its characteristic strong smell and somewhat acidic taste.
When opened, the contents release a strong and sometimes overwhelming odour; the dish is ordinarily eaten outdoors. According to a Japanese study, a newly opened can of surströmming has one of the most putrid food smells in the world, even more so than similarly fermented fish dishes such as the Korean Hongeohoe or Japanese Kusaya.
The Baltic herring, known as strömming in Swedish, is smaller than the Atlantic herring, found in the North Sea, and traditionally the definition of strömming (Baltic herring) is herring fished in the brackish waters of the Baltic north of the Kalmar Strait. The herring used for surströmming are caught just prior to spawning.
At the end of the 1940s, producers lobbied for a Royal ordinance that would prevent incompletely fermented fish from being sold. The decree forbade sales of the current year's production in Sweden prior to the third Thursday in August. While the ordinance is no longer on the books, the retailers still maintain the date for the "premiere".