While no one is completely sure exactly why you’re not supposed to wear white after labour day, or when or why this fashion rule came into effect, the best guess is that it had to do with snobbery in the late 1800s and early 1900s.
In the United States, the wives of the super-rich ruled high society with an iron fist after the Civil War. As more and more people became wealthy, it was difficult to tell the difference between respectable old-money families and those who only had ‘vulgar’ new money.
By the 1880s, in order to tell who was acceptable and who wasn’t in high society, the women who were already “in” felt it necessary to create dozens of fashion rules that everyone in the know had to follow.
Not wearing white outside the summer months was one of those silly rules. White was for weddings and resort wear, not dinner parties in the fall. Of course it could get extremely hot in September, and wearing white might make the most sense, but if you wanted to be appropriately attired you just didn’t do it.
Image source: https://www.britannica.com/biography/Coco-Chanel