On November 20, the new-and-improved Museum of Modern Art will open its doors to the public. But on Monday evening, a lucky few – about 260 trustees, senior museum staff, high-level donors, and others – got an early glimpse of the soaring atrium designed by architect Yoshio Taniguchi.

The celebration marked the exact date of the museum’s founding 75 years ago. Back in 1929, MoMA’s first director, Alfred Barr, had three patrons to thank for setting up the museum in rented rooms on Fifth Avenue at 57th Street: Abby Rockefeller, Lillie Bliss, and Mary Quinn Sullivan. To say the list of donors has grown would be an understatement. Director Glenn Lowry had many to thank for the museum’s expansion – including the city, which contributed $65 million to the project.

“This is such a significant project for public support,” said the commissioner of the department of cultural affairs, Kate Levin, who attended the party, as did Mayor Bloomberg.

“For one, the museum will attract hundreds of thousands of visitors each year, which will have an immense economic impact on the city. But even more important is the nourishment of artists and their audiences. They are essential to the creative lifeblood of New York City and help create its unique identity throughout the world,” she added.

At the event, the son of one of the museum’s founders and an integral trustee, David Rockefeller, spoke of his memories attending MoMA’s very first opening, when he was 14. He grew up with the museum, as a collector and as a philanthropist – the Rockefeller family donated the museum’s first true home, a townhouse on West 53rd Street.

Another guest with long memories of the museum is Kitty Carlisle Hart. “It’s hard to believe the museum is 75. I’ve known it for so long. We’re both doing well,” she said. Mrs. Hart, who raved about the new building, did not, however, sing “Happy Birthday” on Monday night. “They didn’t ask me. That’s a shame. I should have thought of it.”

In the end, the evening belonged to Mr. Lowry, who led a Champagne toast during dinner (risotto and roasted quail; the tables and the flowers were white – so as not to overwhelm the art…. hardly possible with Monet’s “Waterlilies” in the room).

The entertainment was a 12-minute film, which loosely told the story of the museum’s building evolution.

The chief curator of the department of film and media, Mary Lea Bandy, created the film at the request of Mr. Lowry, who asked for something “full of esprit.”

Ms. Bandy chose to tease Mr. Lowry by representing him with clips of Cary Grant, Buster Keaton, even Babe the Pig. No trustees were portrayed (“I’m not that stupid,” said Ms. Bandy over the telephone yesterday).

The film included a clip from a 1939 newsreel of the museum’s International Style building (which cost $2 million) and footage of Mr. Taniguchi, walking through the latest construction.

There were several sweet endings that night. In addition to a dessert buffet in the lobby, there was a ceremonial cake cutting. Several trustees used knives engraved for them to slice into chocolate cakes, each featuring renderings of the building’s facade in 1932, 1939, 1953, and 1984. (Bill Schutz of Creative Cakes is next creating a cake in the shape of a MoMA tour bus for the International Council’s party this weekend).

The event was the first bash in MoMA’s new home (not counting the staff dinner for 2,500 on Friday), and it kicked off two weeks of parties including an opening night gala for a few thousand on November 18.