Who says you can’t wear white after Labor Day? On Saturday evening, 2,000 guests shunned the idea that doing so is a fashion faux pas — and looked good doing it — when they converged on the Rose Kennedy Greenway for the fourth annual Boston edition of Le Dîner en Blanc. A mass “chic picnic,” as it has been called, the event — which was launched in Paris nearly three decades ago and now takes place in over 70 cities worldwide — invites guests, decked out it white from head to toe, to an elegant pop-up picnic in a public city space (past local editions have taken place at City Hall Plaza, the Harborwalk in front of the Boston Children’s Museum, and Moakley Federal Courthouse). One catch? The location is kept secret from attendees until the very last minute to build anticipation. The hosts of this year’s sold-out Boston evening were Samantha Kanter, Anna Berezina, and Kelly Early. Once they arrived, guests set up their own tables and settled in for dinner, complete with champagne and wine (only white wine, of course), before they hit the dance floor. Très chic, indeed.