Don't wear white after Labor Day! (even though the last day of summer is technically September 21st).
The tradition began back in the 1800s, as a way to use fashion to separate those with money from those without - those who needed to work from those who didn't.
When summer was over, affluent residents would leave the city for warmer vacation spots. Wearing white represented that you could afford to get out of the city and vacation elsewhere when summer ended.