THERE are picnics, and then there are picnics.

Three weeks ago, in the golden light of an early-summer evening, thousands of Parisians dressed entirely in white converged on two of the city’s most picturesque locations — 4,400 of them in the plaza at the cathedral of Notre Dame; 6,200 in a courtyard of the Louvre — for a feast that was neither advertised nor publicly heralded. They had brought along not only their own epicurean repasts but also their own tables, chairs, glasses, silver and napery.

At midnight, after dining and dancing, they packed up their dishes, stowed their empty Champagne bottles in trash bags brought for that purpose, stooped to pick up their cigarette butts from the cobbles and departed. The landmarks were left immaculate, with no traces of the revelry of the previous three hours.

This annual event, called the Dîner en Blanc — the “dinner in white” — is like a gustatory Brigadoon, equal parts mystery, anachronism and caprice. Now attended by thousands at some of the best-known Parisian spaces, it began humbly in 1988. That year, François Pasquier, now 67, returned to Paris after a few years abroad and held a dinner party to reconnect with friends. So many wanted to come that he asked them to convene at the Bois de Boulogne and to dress in white, so they could find each other.

But while in certain circles in Paris, everybody knows about the Dîner, many Parisians have never heard of it. And despite the precision that goes into its planning, it retains an air of surprise.