How do you know summer is pretty much in full swing out in the Hamptons? The Parrish Art Museum has its annual Midsummer Party, which brings out a mixture of artists, their benefactors, the occasional fashion designer and socialite and what seems like much of the community's octogenarian population.

"Every year this is a big deal," said Caroline Hirsch, the owner of the comedy club Carolines on Broadway. "It's a museum that's right in our backyard."

Soon, the Parrish will be right in Ms. Hirsch's backyard. She lives in Watermill, where a new Herzog & de Meuron designed edifice housing more of the institution's permanent collection is set to open next year. Saturday's party was the last to be held at the structure on Jobs Lane.

Was everything on schedule?

"We always try to be on time," said Ben Krupinski diplomatically. He is the local builder who is overseeing the project, visible from Route 27 on every trip back and forth from Bridgehampton, perhaps its best advertisement.

"Anyone who's been stuck in traffic can see it's a pretty great site," said the museum's director, Terrie Sultan.

Herzog & de Meuron, Mr. Krupinski continued, "wanted to put a floating bench in the wall. In the actual wall! I said, 'Do you know how to do that?' They said, 'No.'" But it's all been figured out, he explained, via some process that's too complicated to go into here.

The social crowd came decked out in jewel-toned gowns: Debbie Bancroft in cobalt blue; Tiffany Dubin in purple; BJ Topol Blum in orange; Kathy Hilton in a rainbow Ralph Lauren.

"We mimic the artwork," said Ms. Bancroft, referring to the multihued, three-dimensional, geometrically asymmetrical Dorothea Rockburne paintings on display. Ms. Rockburne, a Canadian painter, is having her first career retrospective at the museum at the moment.

In truth, there was more probably work on the guests on Saturday than on the walls. But this is a testament to a) the self-preservation tactics employed in well-heeled, extremely social neighborhoods like Southampton, as well as to b) the aspirational ladies who come husband-hunting at an event like this, where rich men, they suppose, are in great supply. They are also, they forget, in great demand.

Since the Parrish party falls in July, there is usually a climate problem. Cocktails under the tent are usually too hot—thank you, waterproof mascara—and dinner in the tent, at least until everyone starts sucking up the air, is like a meat locker.