The Museum of Modern Art has a big board, a big building, a big garden, and big parties to match. The party that’s the most fun is the Party in the Garden, a 37-year-old tradition that brings together the city’s most powerful people before they disperse for summer vacations.

This year’s party was certainly big – 850 guests gathered for cocktails and dinner, and 1,000 young ones floated in for the after-party. It was a big occasion, marking the return of the party to its proper home at the museum after three years off the premises while the expansion was under construction.

And yet what made the evening so special was not the bigness of it, but rather the intimacy. The gathering felt almost like a home dinner party.

That had mostly to do with the theme of the party: celebrating David Rockefeller’s 90th birthday.

One didn’t have to know Mr. Rockefeller personally to feel a part of what must have been a gratifying occasion for him. Mr. Rockefeller, whose actual birthday is June 12, grew up with the museum, which had its first exhibit when he was 14. The beloved sculpture garden is named after his mother, Abby Aldrich Rockefeller, one of the museum’s founders.

All eyes were on Mr. Rockefeller as he arrived – or were they looking at his beautiful granddaughter, Miranda Kaiser Duncan, who wore a show stopping Oscar de la Renta gown in teal, and a matching choker that belonged to her grandmother?

As he walked around the garden accepting birthday wishes from friends and strangers, Mr. Rockefeller’s clear blue eyes twinkled. This was a pretty grand way to celebrate. And as the evening progressed, things got even grander.

Just when the heat had taken its toll on freshly pressed tuxedos, the crowds were summoned to dinner in the air-conditioned lobby and atrium – this being the first bi-level dinner, the museum’s board president emerita, Agnes Gund, noted.

Here, the focus on the key celebrant became clearer. The preppy, nautical style of the menu and decorations paid homage to Mr. Rockefeller’s passion for sailing and his home in northern Maine. The menus featured a red, white, and blue flag emblazoned with the letter R, for Rockefeller. The tablecloths were white cotton duck trimmed with grosgrain ribbon. On each table were lush bouquets of hydrangeas – either all white or all lavender/blue – surrounded by votive candles in blue glass chargers. Scattered throughout the room were large vases of mock orange branches, wrapped with white sailing rope. All of those details were carried out by the event designer, Avi Adler.

Birthday meals should be special, and Glorious Food didn’t disappoint, serving a fancy Maine supper: heirloom tomatoes; lobster Thermidor with asparagus and roasted corn, and Maine blueberry cobbler.

There were birthday presents and cake, too. The presents were photographs by William Wegman, presented by the artist. The cake, by Creative Cakes, was a three-dimensional copy of Mr. Rockefeller’s Maine home, including the horse and carriage on the grounds, his two sailboats, and his dog. The icing on the “ocean” spelled out happy birthday to “MoMA’s Maine Man.”

And at the end of the meal, everyone sang “Happy Birthday.”

“Really, every aspect of this evening makes me happy,” Mr. Rockefeller told guests.

Had the evening passed with nary a moment of official museum business, guests would have been content. But there was important business to announce: the appointment of a new chairman, Robert Menschel, and a new president, Marie-Josee Kravis. The muckety-mucks also noted that the event, with the support of sponsor Gucci, had raised $3 million – quite a present for the museum.

The after-party, back in the garden, was an entirely different story. Few of the elders stayed on for the revelry, which departed from the preppy and elegant Rockefeller style celebrated earlier in favor of a nightclub theme, complete with two mirrored disco balls hanging from a tent in the garden. There was plenty of food for the young professional crowd: beef empanadas, dumplings, spring rolls, chicken satay – and no lobster in sight. The music was all over the map, from the Earth, Wind and Fire song “September” to hip-hop and dance hits from the ’80s and ’90s. And this crowd could dance. The random musical guest was the Cowboy Junkies, who gave the evening its few calm and mellow moments.