We've all heard (and probably ignored) this common fashion rule but just where did it come from anyway? Let's take a look.
Let's start, of course, with sweat. In the pre-AC summers of the Guilded Age, cities were a sweaty mess. There were no breezy tank tops to help cool anyone off. People wore, as a general rule, formal, modest clothing which covered more skin. Makes perfect sense then that long-skirted ladies and suited gents would obviously opt for cool, white cotton clothing rather than sun-attracting black. But think about the conditions of many turn of the century American cities: grimy, smoggy and dusty. Not exactly the optimal conditions for wearing your best whites. Wearing white in the city meant you could afford to ruin it - not the case with most people of the day with their sparse wardrobes.
So where could one get away with white? Why, the country house, of course. White clothing implied that you were (or would soon be) out of the city on summer vacation (and could afford to take the summer off) unlike those unlucky factory workers and office drones who had to don a black suit every day in August and report to work.
Gradually, summer white became a symbol of luxury (an idea that fashion magazines of the day fueled with their glamorous spreads) and like other exclusive aspects of high class living, it became a way to differentiate people who could afford leisure from those who couldn't.
So, what about the rule about when to stop wearing it?
White was a summer-specific social statement — one only wore white while having a fabulous, glitzy time summering out of the city. When Labor Day rolled around that meant it was time to leave the country house, return to real life, pack away the whites and don a more formal, dark wardrobe appropriate for city living. The custom solidified into a rule; one that's still embedded into our culture today.