At some point in the early 20th century, it was decided that anyone wearing a straw hat by the end of summer must be a complete chump. As soon as September 15 rolled around, it was felt hat season, m**********r. Young men would slap straw hats right off of people's heads when they saw them -- you know, as a public service. Unfortunately, not everyone appreciated this gesture. In September 1922, some straw-hat-wearing dock workers fought back against a bunch of hat-slapping kids, and things escalated from there.
Soon, packs of hat haters were ambushing pedestrians and even jumping on motorists, while "a mob of 1,000" burned piles of stolen straw hats in large bonfires. They were armed with clubs with nails on them. It was like a Mad Max movie by way of Charlie Chaplin.
The violence died down after a few days, but the message was loud and clear: Do not wear a goddamn straw hat after September 15. The problem sort of solved itself once the Great Depression came around, since no one could afford hats anymore.