The Ball on the Mall is the only gala hosted on the National Mall. Here, the scene from outside the National Museum of American History, looking towards party. ( Daniel Swartz/ )

Veteran socialites know Washington’s event spaces as well as their own shoe closets: Time and again, they return to places like the National Building Museum, the Mellon Auditorium, the Washington Convention Center and the big hotel ballrooms for the galas and benefits that dominate the social calendar.

So there’s something unexpected yet jarringly familiar about the site of the Ball on the Mall. Guess where they hold that one? We just told you — literally, on the Mall.

If you drove past on 14th Street Saturday night, you heard what sounded like a pop-up nightclub and saw what looked like a wedding on steroids. That was the fourth annual Ball on the Mall. Under a massive white tent, about 1,000 young-ish Washingtonians in black tie and gowns dined and danced and took in the view — the Washington Monument at one end, the Capitol on the other, the supermoon peeking out from behind clouds now and then.

When Marissa Mitrovich first attended the event, “I was just floored!” she told us. “It’s a ball, on the Mall.”

Inside the massive tent erected by the L'Enfant Society and the Trust for the National Mall for the fourth annual Ball on the Mall. ( Daniel Swartz/ )

The crowd: What Mitrovich described as the “next generation of philanthropists,” i.e., under-40s; as many of them in fancy-dressed packs of roving singles, it seemed, as were there with dates.

The music: Shifted from Michael Bublé to Beastie Boys as the dance floor filled up.

The bar: Open and free-flowing though crowded.

The bathrooms (you were wondering, weren’t you?): No Earth Day porta-potties, thank goodness, but luxe trailers trucked in for the occasion.