Time to pack away the white t-shirts, shoes, shorts, and skirts, right?
While this is more a question for Tim Gunn than for Emily Post, we'll add our two cents to the discussion.
Back in Emily's day—the nineteen 00s, 10s and 20s—the summer season was bracketed by Memorial Day and Labor Day. Society flocked en masse from town house to seaside "cottage" or mountain "cabin" to escape the heat. City clothes were left behind in exchange for lighter, whiter, summer costumes. Come fall and the return to the city, summer clothes were put away and more formal city clothes donned once more. It was an age when there was a dress code for practically every occasion, and the signal to mark the change between summer resort clothes and clothing worn for the rest of the year was encapsulated in the dictum "No white after Labor Day." And it stuck.
Of course you can wear white after Labor Day, and it makes perfect sense to do so in climates where September's temperatures are hardly fall-like. Even in the dead of winter in northern New England the fashionable wear white wools, cashmeres, and down-filled parkas. The true interpretation is "wear what's appropriate—for the weather, the season, or the occasion." So on a blistering Indian Summer September day here in Vermont, you’ll catch us wearing our “whites”.