Nobby dreamed that Saturday night suddenly could be so chic. "Usually," said veteran party-goer-giver-planner Ann Willoughby, "we save Saturdays for the family."

Yet there they were, more than 200 of Washington's most fashionable, forsaking Saturday night home fires for the arctiv blasts of 18th and K Streets and a black-tie dinner at the International Club to honor a bunch of movie stars.

"The magic word," said one guest, " is Bagley."

And indeed, after an 18-month-long rise from comfortable, well-to-do Winston-Salem obsurity to virtual stardom in Washington, D.C., society, Smith and Vicki Bagley's name did probe to be alluring.

The result, in effect, was that the party became the Bagleys' debut as hospitable philanthropists, a social duo (with Reynolds Tobacco wealth and status as early Carter supporters) to be reckoned with in the Carter years ahead.

In one week's time, the Bagley name on a couple of hundred standard fill-in-the-blanks embossed invitations, had succeeded in bringing to gether under one roof as eclectic a gathering of social and official Washington as some of them could remember. Two hundred had been expected; 240 showed up.

To help ease their jet lag, the dinner also was arranged to introduce the dozen or so Hollywood stars, producers and director Robert Aldrich of the new $6.5-million suspense film, =Twilight's Last Gleaming," to the real-life "cast" of the movie's locale - Washington.

It was a composite guest list drawn up from the world of films (Richard Widmark, William Marshall, Charles During, Joseph Cotton); media (Joe Alsop, Walter Ridder, Edward P. Morgan, Austin Kiplinger); diplomacy (Japan's Ambassador Togo, Israel's Dinitiz, Pakistan's Yaqub Khan); Congress (Sens. Edward Brooke, and Carl Curtis, Reps. Ron Dellums, William Cohen, Robert McClory); the administration (Frank Moore, Mary King, and White House counsel Robert Lipschutz); labor (Frank Fitasimmons, president of the Teamsters); and society (Evangeline Bruce, Yolande Fox, Cherif Guellal, Rose Marie Bogley, Susan Goldwater, Mieke Tunney).

Washington, said Emanuel L. Wolf, chairman of the board of Allied Artists, which is spending $4 million to release the film around the world, is "where it's happening."

Author of a play which she is optimistic about getting produced, Mrs. Bagley's Hollywood connection goes back to her single days in Southern California. "I used to date a movie director," she explained.

She did not identify him, but Holly- wood being like Washington in these things, one's former associations are often justification enough for one's present endeavors, however remote they may seem to thers.

The Bagleys met some of their honored quests for the first time ove dinner Friday night at the Palms so that by Saturday night, not all were utter strangers. Some arrived iwth wives, some brought their mothers, and actor Burt Young even brought daughter Ann, 10, who stopped traffic on the dance floor after dinner when she did a very grown up two-step.

The benefit premiere of "Twilight's Last Gleaming" at the Kennedy Center last night (proceeds go to the national Press Club's Press Foundation) had been a sell-out for several weeks. The dinner party was an afterthought and when the show's producars and NPC sponsors added it to the weekend activities, retiring NPC President Robert Alden asked the Bagleys to serve as hosts. They accepted despite plans to move on the same day from the guest house into the main house of their recently purchased $650,000 Georgetown property. (Saturday was also Vicki Bagley's birthday, "alsong with Bob Alden, Adlai Stevenson and Hank Aaron," confided Alden.)

Everyone assumed the Bagleys were paying for the party until manny Wolf told several guests: "I'm paying for everything." When the bar ran out of white wine and one guest kidded Wolf about it, he replied that he wasn't in charge of arrangements, only picking up the check.

How much Allied Artists was spending on the total weekend promotion package, he said, was a "minor" amount until Jayne Brumley Ikard, in her best ex-Newsweek correspondent manner, lectured him into full disclosure by reminding him of the film's "truth in government" theme.

"About $100,000," said Wolf, shiipishly.

Yesterday. Ed Siegenfeld. Allied's vice president, said that "it wasn't Allied Artists" who paid for the party, adding with a chuckle that Wolf "ought to look at the budget closer - that was a private party."

Last night's benefit at the Kennedy Center, on the other hand, was "our party," and while expenses could go as high as $100,000 it was a subject Siegenfeld wasn't particularly anxious to discuss. "We do get something out of it," said. "We fell it (the film) has something to say, and that this is the place to open it."

If who paid what was confusing, so was who went where. At first, the dinner was planned for the NPC ballroom but according to one insider, warning signals sounded when party organizers decreed that the NPC bulletin board had to go, the news ticker had to go and all 50 state flags had to come down. The final straw apparently came when efforts by the organizers to have a "tasting" - a preview of the menu - could not be arranged with the press club chef. After that it was goodbye NPC, hello International Club.

Dinner organizers modbye NPC, hello INternational Club.

Dinner organizers moved swiftly, telephoning many on the list, sending revised ivitations to others. But mixups were inevitable and by 9 p.m. Saturday, the Bagleys still were receiving stragglers returning from the wrong address.

Both Bagleys appeared devastated by the snafu and apologized repeatedly to guests who had gone astray. Most took it in good stride, delighted to be in from the cold in more ways than one because as one of them said, "Lost of people have been anticipating a lot, wondering what the Bagleys were all about."

Such an array would have over whelmed most Washington socialites but at least one member of the NPC benefit committee was lamenting the absence of Carter appointees.

"Nobody knew where to send invitations to the new administration," she said. "And since many wives aren't here yet, the men just don't know what social commitments they should be making."