Men in the early 20th century who wore straw hats after September 15 were commonly ridiculed. Their hats were snatched off their heads and crushed. During one fateful year, men in straw hats were physically assaulted by mobs of teenage hoodlums in New York City.
Men commonly wore hats in the Roaring Twenties, but societal norms strictly prevented them from donning the straw variety after September 15. No one’s exactly sure why that was the cut-off date, particularly since summer doesn’t officially end until September 21.