Now that your Marysia scallop bikini has been packed away, you can turn your attention to more seasonally appropriate things: spooky vibes and this whole "no white after Labor Day" business.
To save you from opening a 74th tab to Google the answer, we
opened a 182nd tab in our second window and found it for you. Prepare yourselves—the truth is kind of lame.
In the late 19th century—long before you could wear jeans to a Michelin-starred restaurant and not be greeted with the maître d's scornfulest scorn—the society ladies were engaged in an invisible battle with the nouveau riche, one that could only be won by the subtle manipulation of fashion. So they telegrammed one another like, "Gertrude. Girl. Why don't we settle on an arbitrary date on which we all stop wearing white for the summer? To separate ourselves from the new-money heathens, you know?"
Gertrude and the rest of them spat on their hands and shook on it, and eventually, when Labor Day became a federal holiday in 1894, that was when you knew to put away your linen suit and spend the rest of September schvitzing silently. But properly.
A bit disappointing to find out that this thing that is simply Not Done evolved from one of those petty distinctions in dressing that was invented to ID the Gatsby-ish interlopers ::eyes roll back in head so hard::, but that's history! Besides, no one GAF now, what with winter whites and summer furs and Vetements charging $1,300 for a hoodie. And we're definitely better off for it.