The president and Mrs. Reagan began a marathon of ball-going last night with a visit to the American Legion's banquet in a capital celebrating both the inauguration and the hostages' freedom.

"Where's Jimmy? Where's Jimmy?" said Nancy Reagan as she bussed friend Betsy Bloomingdale on both checks at the Kennedy Center Inaugural Ball. It wasn't Jimmy Carter she was looking for but designer James Galanos, who had created her show-stopper one-shoulder white gown.

The Reagans made a short appearance at this, their sixth ball. Mrs. Reagan, with a new pulled-back hairdo, also gave Jerome Zipkin a two-cheek kiss -- "Did you get the package?" she asked him across the velvet ropes. "Can you imagine she thought of that?" glowed Zipkin, who wouldn't reveal what the package contained.

"I want to see you in that dress again before it goes to the Smithsonian," said Betsy Bloomingdale as the first lady left with the president.

It's doubtful that Jimmy Galanos ever got a full-length view of his dress, as he was caught in the massive jam in the foyer of the Kennedy Center.

Barbara Bush had better luck connecting with her designer. "Am I doing everything correctly?" she asked Bill Blass, who created her blue, hammered-satin gown and chose her pseudo-sapphire necklace. "Glorious," said Blass. The vice president agreed. "Elegant," he said.

"I don't like big crowds," said Roy Rogers, who was wearing black cowboy boots. "I'm just a country boy. Anything over six people scares me." cRogers, who was with his wife, Dale Evans, said he visited four of his restaurants here during the day.

The ball was a good mix of Halston glitz and polyknits -- and only one tiara. The tiara was fake, but Harry Platt, newly elevated chairman of Tiffany's, was sure lots of the other jewelry was real.

Said one woman, peering down into the waist-high curtained booths that lined the rooms in the Kennedy Center: "They look like cattle in their $2,000 stalls."