It's a dictum that originates from my earliest years, from the same place as skirt and sweater sets and little white gloves for dancing school. From a time when there were rules and penalties (a withering glance, an averted head) for breaking them.
Those have gone away, of course — the rules and the penalties — but wearing white after Labor Day ... Well, that's a tough one to break. So white skirt and pants are tucked away for next year. White blouses and shirts, they're allowed, of course. I'm wearing one right now. A way to keep the flickering flame of summer burning brightly a few more weeks.
(In no hurry for this kind of white.)