This was the triumphal inaugural weekend that was: velvet ropes and jockeying limousines and furs flying as the Reagan era, engraved invitations in hand, partied its way into the frigid heart of the nation's capital. The social countdown began Saturday, with the hope of a hostage settlement always in the wings. And, after Ronald Reagan is launched into the presidency tomorrow, the celebrations will continue well into the night. In service to this lavish, decorous revelry, a multitude of hotel rooms, Georgetown salons and vast banquet halls have thrown open their doors for a social honeymoon that puts suntanned incoming Californians, homeward-bound Democrats and real, live movie stars in the same canape lines. Eat, drink and be merry, all. For Wednesday, the loyal opposition reforms.

"I never declare presidential preference," grinned Johnny Carson, king of late-night television and inaugural gala emcee-to-be.

"You gotta have fun with it. After all, it's not a coronation."

A lift of the eyebrows.

"Is it?"

Probably not, but not for lack of trying. Lavish is the word that best describes the buffet bash NBC president Fred Silverman hosted yesterday at the Four Seasons Hotel. The crowd was a rarefied mix. From NBC, celebrities and bigwigs -- Carson, Ed McMahon, "Today" host Tom Brokaw, NBC News president William Small among them. And actor Robert Conrad, looking like Tom Mix in a huge tan Stetson.

From the next White House, Reagan Chief of Staff James Baker, National Security Adviser Richard Allen and Vice President-elect George Bush, who wore an NBC peacock lapel pin and moved about in a throng of stonefaced Secret Service men.

"This is the most attention he'll have for the next four years," said an irreverent Washingtonian.

There was also a veritable quorum of senators, including Robert Dole (R-Kan.), Charles Mathias (R-Md.), Joseph Biden (R-Del.), Barry Goldwater (R-Ariz.) and Henry Jackson (D-Wash.), and Reps. Jim Wright (D-Tex.) and Toby Moffitt (D-Conn.).

"It's a good party," said Silverman. "Isn't it a good party?"

Others spotted in the crowd: David Brinkley, Roger Mudd, Elizabeth Drew, Marvin Kalb -- or was it Bernard? -- and "Today" anchor and cheerful presence Jane Pauley with her husband, cartoonist Garry Trudeau.

Silverman was hovering by Carson as Henry Kissinger, man of a thousand party invitations, walked into the room. "Say, Henry, there's a party . . .," said a man gliding over to extend yet another.

"It's my charm," explained Kissinger moments later.

Kissinger, Carson and Silverman converged. A long microphone sprang out of an NBC soundman's hand. TV lights blossomed. The crowd leaned in.

"Don't stop!" squawked a cameraman. "They're talking about the hostages!"

Wrong. They were laughing at each other's jokes.

Sen. Goldwater was in another, quieter, room. "Ostentatious," he growled, when asked for his impression of the inauguration thus far. "I've seen seven of them.And I say when you've got to pay $2,000 for a limousine for four days, $7 to park, and $2.50 to check your coat, at a time when most people in this country just can't hack it, that's ostentatious."

"You can tell he's antisocial," said his wife sadly.

A familiar laugh rolled through the ballroom. Ed McMahon, naturally. He patted Kissinger's shoulder."Now Henry," he said, "if you need any advice at all, I'll be here through Wednesday."

"Ah," said Kissinger.