Fashion Nazis have long considered it to be the height of poor taste to wear white after Labor Day until Memorial Day. It is gauche. It is a fashion faux pas. Of course this begs the question, why? Some might take the question even further and ask, and why are we surrendering to the French and their ooh la la haute couture, anyway? While the latter may have to remain a mystery a little while longer, the former has an answer, albeit a complex answer that has as much to do with good manners and proper social behavior as it does with practicality.
Wearing white mens t shirts in the summer time can be substantially more comfortable than wearing dark, three-piece suits and the same consideration goes with a pair of white shorts or a white t-shirt for women. When the sun is at its hottest and beats down on us in a most oppressive way, the color white will reflect that heat rather than absorbing it, which is what darker colors do, so wearing white in a hot climate actually keeps you cooler, and the thin cotton of mens t shirts allows air to seep through the fabric more easily than a thick wool sports coat, so that you stay cool (even if you are not actually French). Traditionally, Memorial Day signals the onset of hotter temperatures and Labor Day signals the beginning of a slow cool down in the United States, so this fashion rule has a practical component.
That is not the sole reason, however, that it is considered poor taste to wear white after Labor Day. When the national holiday was adopted, after the Pullman strike saw several workers killed in 1894, it was the practice among the wealthier class to leave cities and spend summers in the country. Accompanying this move was a change in wardrobe into mostly white suits and dresses. As a result, white clothing came to be associated with privilege and the ability to take vacations while the working classes slaved along. The notion that wearing white after you returned to the city was bad form originated in the notion that such dress was akin to flaunting one’s privileged status toward the masses. So when the different classes came back together in the cities in the Fall, wealthy people would change to darker clothing to be polite.