"I haven't sung this early since I was a choirboy," said Frank Sinatra as he hopped onto the Regency ballroom stage at the Shoreham hotel yesterday shortly after 1:30 Sinatra was the surprise entertainment for the annual Congressional Club luncheon honoring the first lady.

Hardly a patent-shod foot was still as Sinatra sang for more than a half hour, directing many of the songs to the head table where Reagan smiled and nodded her head to the music. As he finished "New York, New York," she passed her black handbag to Peter McCoy, her chief of staff, slipped off the dais and headed to the stage to greet the performer. After one false start when the song wasn't over, she racedup and threw her arms around him.

"I don't know who is responsible but I can't thank you enough," she said to the audience. "And Frank, I can't thank you enough," Mrs Reagan said.

It may have been early for Sinatra, but present and former congressional wives and their guests started to gather almost an hour before the doors opened for the sellout benefit billed as "Nancy Reagan's Day . . . A day of brightness and joy dedicated to our First Lady . . ."

Once most of the 1,300 guests were seated, special guests -- including former club presidents, Supreme Court justices' wives, Cabinet members' wives, Barbara Bush and Nancy Reagan -- walked the length of the room on a huge replica of the Golden Gate Bridge that almost touched the crystal chandeliers. As the Marine band played, red-jacketed Marines escorted everyone across the wood-and-rope structure to the head table. Roses of mixed colors in trellis baskets were used as table centerpieces and flower-filled blue window boxes with satin streamers edged the front of the dias.

Barbara Sinatra, who ducked behind her table's centerpiece to keep out of view of the first lady until the singer was introduced, said she and her husband hadn't seen Mrs. Reagan since "that unfortunate time" of the assassination attempt when they flew to Washington that very afternoon to be with her. Sinatra's guest appearance yesterday had been plotted months ago with Norma Lagomarsino, wife of Rep. Robert Lagomarsino (R-Calif.) and current president of the club.

Sinatra was introduced as a "gift" to Mrs. Reagan. "I've been called a lot of things," he told the audience, "but never a gift."

Entertainment is always a surprise for this luncheon; in recent years Minnie Pearl, Bob Hope and Robert Merrill have performed. But if club members had trouble remembering who showed up when, they had little trouble remembering the time a first lady didn't show up. It was Jacqueline Kennedy who excuses herself two weeks before the luncheon, according to Jerry Miller, whose husband, now a judge, was a senator from Iowa at the time. The club president complained to Presidnet Kennedy and he came in his wife's stead, soothing the women, recalled Miller, by saying something like "Many of you who are mothers will understand . . ." He was referring to Mrs. Kennedy's being pregnant at the time. The club members also haven't forgotten that later the same day Mrs. Kennedy took a plane to New York for the ballet.

Yesterday's guests were rewarded for their attendance, which benefited the local Florence Crittenton Home, Hospice Care and Big Sisters, with navy fabric shopping bags stenciled with "Nancy Reagan's Day" and stuffed with loot. Included were jelly beans, Godiva chocolates, soaps and perfumes, an umbrella and marmalade. The bags were tied by a yellow scarf to the back of each chair, ready for the guests before they sat down to the luncheon of California gazpacho supreme, Rodeo Drive breast of chicken salad and desert ranch mousse.

The club's gift to Nancy Reagan was a framed needlepoint tapestry of the White House with a background and frame in "Nancy Reagan red."

"I'm getting my sea legs back," said Mrs. Reagan, active publicly again, "and I'm glad to be getting them with you."

"Do you want to go on the road with the act?" quipped Sinatra after he sang, and took Mrs. Reagan's hand, leading her back across the Golden Gate Bridge and out of the rooms.