President Ronald Reagan had the prince of Wales over for dinner Saturday night, and afterward gave him some fatherly advice on a subject that has built great empires and ruined mighty men: marriage.

"The step you're about to take is really a very serious step," one guest quoted the president as saying in his toast to the prince, "but your sense of humor will carry you through."

The prince, who left yesterday for England, responded with thank yous and jokes -- one of them, according to a guest, being a reference to "Lady Diana as 12 his junior, and Mrs. Reagan as 12 years the president's junior." This particular reference would make the first lady 58 years old.

Saturday's dinner was small, private and a topic of fascination at other weekend parties throughout town. Everyone wanted to know who was invited. Only 30 were.

At four round tables of eight each in the second-floor President's Dining Room in the family quarters, you could have found an unusual mixture of Hollywood and London. It was accented with Nancy Reagan's favorite decorator and one of her chosen designers: Ted Graber and James Galanos. Graber, the decorator, stayed Saturday night at the White House. gThe first lady wore a Galanos -- dotted blue, two-piece, long sleeves.

The guest list was decided primarily by Nancy Reagan, although the White House had asked Buckingham Palace for suggestions. They came up with songwriter Sammy Cahn.There were also Anglophiles like Evangeline Bruce and the Paul Mellons, but others, like Jerry Zipkin, came simply as close Reagan friends. Perhaps because the prince is engaged to Lady Diana Spencer, there were no young women -- except for ex-"Charlie's Angel" Shelly Hack, who came with singer Bobby Short. It wasn't the kind of a crowd the prince is accustomed to back home.

Others among the guests included editor William F. Buckley, designer Diana Vreeland, British Ambassador Nicholas Henderson and Lady Henderson, Cary Grant and Audrey Hepburn.

"Very cozy evening," Bobby Short said afterward. "It was choreographed brilliantly."

The evening began a few minutes before 7:30 when the prince arrived. First. At state dinners, the guest of honor almost always arrives last so he or she can enter the White House to see a crowd of people ready with greetings. But because this was a private dinner, the Reagans apparently decided they would to it differently.

"The dinner went exactly as it was planned," said White House social secretary Muffie Brandon. "He arrived exactly on schedule."

As Prince Charles' limousine drove up, a crowd of protesters outside the White House gates chanted "Bobby Sands! Bobby Sands! Bobby Sands!" They were referring to the Provisional Irish Republican Army prisoner who is two months into a hunger strike. The prince had no visible reaction.

He was greeted on the North Portico by East Wing staff director Peter McCoy, then taken inside. The Reagans waited from hi at the top of the stairs. Four minutes later, the rest of the party began arriving. Hardly anybody was late.

In fact, Betsy Bloomingdale, a longtime friend of Nancy Reagan's, was so concerned about being on time that her limousine actually arrived outside the White House gates five minutes before the set time of arrival.

So Bloomingdale, sharing the limousine with her husband, Alfred, as well as Audrey Hepburn and her date, Robert Wolders, directed the driver to go around the block. "At 7:25 p.m.," she recalled, "we were cruising around the White House." The stall saved them. They managed to arrive safely in fashion, just a few minutes after the half-hour.

Dress for the evening was black tie, but can better be described as Bill Blass and Adolfo. Almost all the women wore designer gowns, heavy on purple and black and giant, puffed sleeves. Some sleeves were so big as to suggest that dining next to the sleeve-wearer might have been something of a challenge. tBut they're very in, and that's all that matters.

The menu: fresh aspargus and crab mousse in aspic, fine herb sauce and cheese twists, saddle of lamb with mint sauce, braised fennel and green beans, and "Crown or sorbet Prince of Wales." That's a red, white and blue ice cream mold -- red respberry sherbet, white coconut ice cream and blueberry sherbert.

After dinner, Bobby Short played piano pieces like "Room With a View" and "Honeysuckle Rose" in the yellow Oval Room. Everyone got a chance to talk with the royalty. The president talked with him about acting, and Betsy Bloomindgale chatted about English tea. In this kind of an evening, politics is afterthought.

Naturally, everyone invited raved about the evening and spent yesterday morning on the phone with friends and the professionally curious who asked: "SO HOW WAS IT??"

"Delightful," said publisher Walter Annenberg, former ambassador to Great Britain.

"Small and lovely," said Cary Grant's wife, Barbara.

"One of the most spectacular evenings I've ever spent," said Jerry Zipkin. His line was busy a fair portion of the morning.