Straw Hat Day
Straw Hat Day is held on May 15. This event in the second decade of the month May is annual. 
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The origin of Straw Hat Day in the United States is not clear. Jack Lambert, owner of perhaps the busiest hat store and mail-order hat company in the country, JJ Hat Center in New York City, said it probably was a promotional gimmick of the hat industry in the 1920s.
Baltimore companies manufactured so many straw hats, particularly boaters, in the 1940s that the city was known as the straw-hat capital of the country. That distinction ended in the 1960s as the popularity of hats waned.
Source: baltimoresun.com
Similar holidays and events, festivals and interesting facts
Chicken Dance Day on May 14 (“Der Ententanz” was the name of the song that inspired all of this, and while the dance has come to be called “The Chicken Dance”, the song itself is called The Duck Dance);
Nylon Stockings Day on May 15
Biographers Day on May 16
Love a Tree Day on May 16
Sea Monkey Day on May 16 (They’re known as Artemia New York Ocean Science and go through anhydrobiosis, or hibernation when they are dried out. Then, with the right mixture of water and nutrients they can spring right back into life);
Wear Purple for Peace Day on May 16
World Embroidery Day or International Vyshyvanka Day on May 16 (Celebrated on the third Thursday of May. Festival of culture of the East Slavic traditional embroidered shirt. Usually, an embroidered shirt was made from a homemade cloth, which was woven on a loom);
Scratch Day on May 16 (date for 2022)