It's the eve of the inaugural eve, the eve of tonight's big gala at the Kennedy Center. Time has almost run out of rehearsals before they've even begun. But everyone assembles - 170 stars, 200 musicians, 230 technicians - and, somehow, the final run-throughs seem to work. Maybe it's because they're professionals.

Like Chevy Chase.

Chase and colleague Dan Aykroyd, in their own peculiar fashion, will give President-elect Jimmy Carter a sneak preview of his swearing-in ceremony.

"After I swear you in, maybe we should do the traditional torching of the hat," suggests Chase to Aykroyd, as those around them cast anxious glances.

"We can get one of those pre-set plumber's propane torches. You just turn a knob and light it," says Aykroyd in glee at the prospect.

"Don't worry," Chase assures the nervous director and technicians. "We do all our scripts this way. We get a copy just before we go on."

The lines of the skits were embargoed for use from the rehearsal yesterday. But it should be enough to say that Aykroyd appears in the role of Jimmy Carter as he has on the NBC "Saturday Night" show. And Chase, who stumbled his way into hilarity in pratfalls as President Ford, now switches to the role of the Chief Justice of the United States.

Millions will be able to hear the lines tonight when CBS carries the first televised inaugural concert at 9 p.m. The 2 1/2-hour show will be a delayed broadcast of the live show beginning at 7 p.m. in the Opera House. It will be sent on line to New York City to be taped to allow minor editing and the insertion of commercials.

Some 600 stars, musicians and technicians have contributed their talents (the stars like John Wayne and Joanne Woodward get about $700, the union minimum for a 2 1/2-hour show) to give the new President a night of light-hearted entertainment before his swearing-in on Thursday.

Among those appearing will be Bette Davis, the Alvin Ailey Dancers, Loretta Lynn, Paul Simon, Paul Newman, Jack Albertson, the National Symphony Orchestra, Muhammad Ali, Aretha Franklin, Redd Foxx, Freddie Prinze, Linda Ronstadt, the Houston Opera Company stars from "Porgy and Bess" and Shirley MacLaine.

Marty Pasetta looks calm, confident. This is a good thing. His job, in the final stages of preparation, seems like that of a pilot bringing in a large craft under difficult circumstances. He's a CBS-TV director and the director for the concert; all he has to do is keep a couple of hundred people happy and the machinery working. Someone says this always seems to be increasingly difficult as show time nears. But he has a reputation for massaging egos and untangling cable with equal facility.

"Washington," he says, "is a strange city to do things in. It works in its own way. You know, the Secret Service, the government. Show business is very loose. There are fast changes, or any changes."

At least this show is under one roof. He says his next one, to be shown live on CBS Jan. 31 and called "Evel Knievel's Death Defiers," will be produced at 24 locations round the country. In Florida, Karl Wallenda will walk a wire stretched from the Fountainebleau to the Eden Roc; one Orville Kisselberg will attach four sticks of dynamite to his chair and set them off; someone else will jump nine stories into a 3-foot sponge, and Knievel will jump a tankful of sharks on his cycle.

Now it's clearer why Pasetta looks confident about tonight.

There's a crisis with the cue cards.

The cue cards have everything on them. Everything. Like, "Good evening, my name is John Wayne."

These are not little cards, "but large boards. (A man who has just written Paul Newman's entire speech on them is lugging them away with some difficulty, saying, "Okay, Newman's done.")

These boards are big enough that if someone got hit by one he'd know it. And this is the problem.

The cue-card man, who is stationed in the back of the Opera House next to two TV cameras where some seats have been removed, doesn't have enough room to pull back on his cards without hitting someone who will be occupying the seat next to him.

What to do? The cue-card staff huddles and it's decided . . . give the man more room by moving the cameras 18 inches to the left . . . if Marty Pasetta approves. This is one of Pasetta's easier problems. Permission granted, immediately.

There is the biggest cowboy of them all, John Wayne.

"Duke, can you see your lines on the cue cards?" director asks.

"Gee, is that the size of thing? I can't even see the cards," Wayne bellows back.

But the pro knows the lines have been changed on him as he reads his little piece about the swearing-in ceremony at "high noon." He is, Wayne points out, considered a member of the opposition (he supported Ronald Reagan) but it is a loyal opposition.

Wayne, who has a bad cold, doesn't go through a second trial run even though the cue cards are "screwed up" for the lines that he has. No one has any doubt that the Duke will be able to handle the situation on stage.

If you have a bewildered man lost in the corridors of one of Washington's hotel and he calls the telephone operator for directions . . . yes, you have Elaine May and Mike Nichols reunited in one of their telephone-to-the-ear skits for the first time in years.

Nichols and May, who went their separate ways as directors, were back yesterday, perched on the same high stools.

"Elaine looks like she's swallowing the microphone," says Pasetta as the two rehearse. "Can we lower them?"

The professionals know how to protect their performance with attention to such details.

Jean Stapleton, who appears as "All in the Family's" Edith Bunker (she's making a collect call tO Archie from Washington), wants to know whether the lights will be dark when she enters the booth. And, she asks, how about a mask over the lower part of the booth to cover her red gown and show only her Edith coat?

The Bunker skit is a tricky one technologically with Archie's voice recorded in Hollywood, a track added for a telephone operator by an Arena Stage actress, and then Stapleton appearing in person in a phone booth on stage.

Jack Albertson is reworking his lines behind the stage before he goes on to rehearse.

"You know how to exit, Jack," the stage director says at the end to the veteran.

"Just fall off the platform - like Chevy Chase," says Albertson.

Leonard Bernstein comes on with a flourish. He greets the musicians with handshakes, hugs, a kiss for a woman violinist wearing a green button emblazoned with Carter-esque teeth. "Good morning," he says. "Good morning," they all say.

He tells them al he has are "two little songs." They do "Take Care of This House" and then Frederica von Stade comes on to sing it. She is powerful, and good. When she is finished producer James Lipton shouts "Bravo" from the seats of the almost empty Opera House. It's the first time Lipton has smiled.

Then, "To My Dear and Loving Husband," Bernstein's dedication to Rosalynn. "Very good for the first time around," he says. Bernstein will be on for six minutes total.

"They're two slow songs," he says. "That's not supposed to be good technique, to follow a slow song with a slow song." But he says the show is heavy with pop tunes and comedy. "I'm hoping this will be a welcome relief, that's what I'm hoping."

Hank Aaron has finished practicing his introduction of the Alvin Ailey dance group and is backstage signing autographs.

He says he's a fan of Jimmy Carter's, and not just because Carter gave him a Georgia license plate with 715 on it after Aaron, then playing for the Atlanta Braves, had broken Babe Ruth's home run record.

"After I hit the 715th he had my wife and me over to the governor's mansion," Aaron says. "We had a very nice, relaxed conversation." And Carter hinted of his plans to run for President and made a pitch for Aaron's support.

"He said not to get on the other side, that he was planning something big. He talked around it, but I knew what he meant. When we left, my wife said, 'He just announced for President."

Executive producer Lipton has been "depressed the last couple of days." says his wife, Kedakai. "The red tape, you might say. You've got the TV area, the stage area, the inaugural committee, each has its own people you have to go through to get things done. The size of this is so monumental.

"But today, he's excited. He can come to the theater and put it together."

Lipton, a slender, bearded man, talks excitedly. "It's the most intricate operation conceivable with 600 people involved," he says. "Everybody's just getting here. Joanne Woodward is coming on a Concorde now. Everything has to coalesce."

Will it?

"Yes," he says, and seems to mean it.