It happened at one of the queen's summer garden parties on the vast lawn behind Buckingham Place last week. As usual, most of the 6,000 invited guests were watching the terrace of the palace for the first glimpse of the queen emerging from the royal family's quarters. They knew Prince Philip would be with her, and perhaps even Price Charles, who had become an increasingly popular figure at these gatherings of selected subjects of her majesty and members of the dipomatic corps.

When, to the crowd's obvious surprise, Lady Diana Spencer also appeared just behind the queen, the customary silence that greets the apperance of the queen was suddenly broken by quite uncustomary applause. And it didn't stop there. As the members of the royal family slowly made their seperate ways through the throngs on the lawn, exchanging pleasantries, Lady Diana was surrounded by more people than the queen, who had always attracted the largest portion of the crowd in the past.

Lady Diana -- accompanied by Charles, who strolled with his arm around his fiance at one point -- went out of her way to speak randomly to people in the crush around her, in addition to those whom palace officials picked out for small-talk chats with the royal personages. "She was really a true professional," said an onlooker with long experience at court functions. "It is amazing that she has learned to cope to quickly."

In her first public apperance with the rest of the royal family, the tall, slim 20-year-old with an easy smile had stolen the show. A year ago, she was unknown to the British public and was only beginning to get to know Charles well, although she had been acquainted with him since childhood. Now, as the sensation created by her garden party apperacne illustrated, her impending marriage to the prince of Wales has made her this country's hottest celebrity property and added new excitment to Britian's love affair with its millennium-old monarchy.

Lady Diana, who on Wednesday becomes the princess of Wales, is unlikely to displace the queen in the deepest affections of her subjects while she is on the throne. Charles also remains in his mother's shadow as the popualr embodiment of national identity, even though some opinion polls show that people find her personally more likable. But together, Charles and Diana are continuing an evolution and modernization of the world's oldest constitutional monarchy and giving it new verve.

"It will be beneficial for the monarchy to have the limelight on younger people," said Patrick Montague-Smith, former editor of "Debrett's Peerage" and a veteran observer of the royal family. "The monarchy actually has been changing ever since the queen came to the throne. Prince Philip brought a breath of fresh air to protocol. Television and royal walkabouts made the royal family more accessible. So Charles nad Diana are continuing a trend, rather than making a change."

Charles had already continued to this trend by being the first prince of Wales to go off to school and compete equally with his peers in academics and sports, doing slightly better in the latter. He has since combined the picture-post-card glamor of outdoor pursuits like polo, horse racing, flying and diving with long hours of multimillion-dollar charitable work, ernest study and critiques of various aspects of British society, salesmanship for British trade on tours abroad, and frequent contact with Britons and Commonwealth subjects in all walks of life.

Diana, also born to privlege and luxury, also was known more for her determination than intellect at school and has worked at relatively mundane tasks since. "There is a slight tinge of a Cinderalla story here," said Elizabeth Pakenham, the countess of Longford and a biographer of the British royal family. "She had been unknown to the public working as a nanny, teaching at a nursery school and learning to cook [at a cordon bleu school for aspiring chefs]. She seemed, in a way to people, to be 'one of us.' Now she has been transformed into a fairy princess."

yyet, as Montague-Smith pointed out, Diana "was brought up on the royal fringes and was about as close to the royal family as you can get without being part of it. She will blend in absolutely with them."

Lady Diana Spencer was literally born next door to Charles in a house her father rented on teh queen's Sandringham estate on the marshy flatlands of Norfolk in eastern England, before he inherited the family mansion and his father's earldon. She is descended from several English kings and is a distant cousin of Charles. Her family has been moneyed (land and sheep farming) and titled since the 16th century. Various members have served the court over the centuries, with her grandmother a lady-in-waiting to Queen Mother Elizabeth and her brother-in-law a private secretary to Queen Elizabeth II. As a child, Diana knew the queen as "Aunt Lillibet" and played with her younger sons, princes Andrew and Edward. She was still in diapers when she first saw 12-year-old Prince Charles.

But her childhood was disrupted at age 6 when her mother left her father, with a publicly acrimonious divorce two years later. Her fater, accusing her mother of adultery with a wallpaper heir, won custody of the children. Her mother married the wallpaper heir. Diana's life was again disrupted years later when her father married the dynamic duaghter of best-selling romance novelist Barbara Cartland. Diana spent there years in boarding schools and alternated visists with her parents.

She left school at 16 with an insufficient academic record to go further. She hated the finishing school she was sent to in Switzerland, and returned to London to work. She lived with three friends in a $200,000 apartment her father bought in Chelsea and worked first as a nanny and then as a teacher in the Young England nursery school in a chuch hall in London's Pimlico section near Chelsea.

The school's young proprietors and staff have described her as especailly loving and effective with the children. "She would help them make cakes, would get the paint or water ready for their games, and she particularly enjoyed taking them to dance classes near Harrods, where she would help the teacher and dance with the children," said Vicky Wilson, one of the school's owner's. "But she was very organized and domesticated in every way."

By this time, Diana had become reacquainted with Prince Charles at the estate of her father, Earl Spencer, while Charles was dating her older sister, Lady Sarah Spencer, in 1977. Diana, then 16, had corresponded over the years with her childhood friend Prince Andrew, but now found Prince Charles "pretty amazing." In a television interview the day their engangement was announced in February. Charles recalled that he had also discovered Diana to be "a very amusing and jolly and attractive 16-year-old."

they next took notice of each other when Diana, then turning 19, went to the queen's Balmoral Castle in Scotland last summer to help another sister, Lady Jane, with her new baby. Lady Jane had accompanied her husband, Robert Fellows, a private secretary to the queen. Diana and Charles discovered a number of mutual interests from music to salmon fishing and, Prince Charles later told reporters, "we began to realize then that there was something in it."

Charles saw Diana several more times in the autumn at other royal homes and the estates of friends, with "the whole thing planned like a military operation," he said, to maintain their privacy. By all accounts, Charles also avoided discussing Diana with his parents until a royal-family Christmas house party at Windsor Castle at Christmas. They decided to invite Diana to join the family at Sandringham for the new year. By now the media had discovered the romance, and the estate was so besieged by reporters and photographers the queen became angry.

But Diana passed muster. Charles proposed to her over dinner in his small Buckingham Palace apartment in early February. Although he advised her to consider during an upcoming trip with her mother to Australia how her life would be changed, she accepted immediately and their engagement was announced when she returned to England. She spent several days living with the queen mother at Clarence House near Buckingham Palace to hear how the 80-year-old favorite of Charles also moved from mere nobility into the royal family nearly 60 years ago when she married the then-duke of York, who later became King George VI when the last prince of Wales abdicated the throne to marry American divorce Wallis Simpson. At the queen's suggestion, Diana also spent time in the offices, across from Buckingham Palace, of the duchy of Cornwall, the vast dukedom of scattered real estate, farms, mines, quarries and other investments that pays the prince more than a million dollars a year until he becomes king. (Half of that Charles voluntarily gives to the government in lieu of taxes.)

Diana first endeared herself to her new public by the way she stood up to being hounded by the press. Earlier, when the romance was still secret, she told friends her boyfriend was "Charles Renfrew." (Besides a multitude of other titles, Prince Charles is Baron Renfrew.) Once thrust into the limelight, Diana successfully parried reporters' questions without damaging indiscretions before the engagement, and emerged afterwards as a magnetic media personality even though she has spoken few public words. She also endured the embarrassment of her uncle publicly reassuring the kingdom of her virginity. "Lady Diana, I can assure you," he told reporters, "has never had a lover. There is no such thing as her ever having had a past."

But she has especially won over her future subjects with her obviously open and warm personality, her blue-eyed, blond-haired good looks, and with a combination of youthful spontaneity and mature poise and grace.

At another palace garden party yesterday for disabled people, Lady Diana braved a drenching downpour to stay out on the lawn and banter with the guests, many of whom were in wheelchairs. "I hope it stops raining," she said, "though it can do anything it likes this week as long as it's nice next Wednesday."

"I think her qualities are going to be very useful in the royal family," said Kay Seth-Smith, the other owner of the Young England nursery school. "She is very reliable, enthusiastic, very human, has great tact in dealing with people -- in all, an extremely nice person."

"The British public immediately realized her wonderful sweetness and naturalness of character," said Lady Longford. "She always looks charming, even when just watching things, like Wimbledon or a polo match, and she has not the slightest difficulty in saying the right things."

Looking at is as a historian, she said, "it is very like when the queen mother first arrived in the royal family. She also was young, just 23, and she brought gaiety and humor into the royal family. She soon had the older members around her little finger, and she made a world of difference to the then-duke of York.

"I think Diana will be the same," Lady Longford added. "She will lighten life for the royal family and make a terrific difference in Charles' life, which had been getting perhaps a bit static." 4tUniversally pictured at age 32 as a dashing figure, the world's most eligible bachelor, Charles actually had been searching anxiously for both a bride and a better definition of his role during the decades likely to lie ahead before he becomes king. He had tired of the "action man" image conveyed by photographs of his many and sometimes daring athletic pursuits and felt his hours behind a desk, running the extensive royal charities he had built up or painstakingly researching and writing exhortive speeches about British industry and education, were not properly appreciated. He wanted to be certain he was using his privileged position, despite its lack of any real power, to accomplish something.

He also was lonely, according to those who know him personally, confining his social life to a small circle of trusted friends, most of whose lives are just as remote from everyday events as his own. His friendships with women often seemed tentative and fleeting. He spent many nights alone in his palace apartment, reading and listening to music or watching television while eating dinner off a tray brought by servants. Despite a close relationship with his parents, their and his busy schedules precluded many chance get-togethers, aside from the usual holiday periods or special occasions.

Suddenly, there was Diana and everything appeared to fall in place. Besides their mutual interests, both laugh easily, hate smoking, drink only moderately and are naturally shy despite their open public personalities. Both like children. But Diana has said she does not particularly like horses, having fallen off one as a child, and reportedly shares the queen's fears that Charles could someday injure himself badly in one of his frequent falls from polo ponies and race horses.

After the wedding and the royal couple's Mediterranean cruise honeymoon, attention will turn to Highgrove, the mansion in the Cotswold Hills that Prince Charles bought for about $2 million last year. With Prince Andrew, a truly dashing and carefree bachelor, taking over much of his older brother's schedule of public appearances, Charles and Diana are expected to start a royal family of their own and build a comfortable married life for the prince before he plunges ahead with whatever he settles on doing before becoming king.

"Diana will have her own interests, I expect, probably things having to do with children or the disabled," said Lady Longford, "but her business will be to help make Highgrove into a real family home."