It started back in the early 1900's, where the summer season was identified by Memorial Day and Labour Day. Society traveled from their town homes to their summer homes, leaving their "city" clothes behind and taking their "lighter, whiter" summer clothes with them. As fall arrived, their summer outfits were left behind and their city clothes were worn again. The "No white after Labour Day" was the mark to change clothing for the season, and it stuck...for a few :)
So is it okay to wear white after Labour Day? Absolutely!
White can be worn beautifully with fall shades. It brightens things up a bit! The key is to layer your whites. Wear a plaid shirt under a white cotton, cashmere sweater with a pair of jeans. Or... A black pant, earth tone colored top with a white short blazer.
Just remember to watch the fabric and weight of your white clothes. Anything to light or frilly won't carry over well into the fall. Think more of crushed velvet, faux fur, cashmere or a heavier cotton for the the fall/winter months.
White pants are trickier. Definitely put your linen, lighter cotton pants away. If you feel daring enough, a winter white pant / jean can be beautifully worn layered with fall/winter tones. My best advice is to wear what's appropriate for the weather, the season and the occasion. White is absolutely okay to wear after Labour Day when put together right.
Happy white shopping!