Philadelphia ? Oh, un-happy day.

Saturday marked the 25th anniversary of the wedding ceremony uniting Lady Diana Spencer and Charles, Prince of Wales – an event that captured a global audience of 750 million.

Americans turned on their televisions as early as 5 a.m. to witness what seemed to be a fairy tale come true: A shy, almost gawky 20-year-old had been plucked from obscurity to marry the heir to the British throne. She would be a princess and, one day, queen.

But the dream started to unravel even before the couple returned from their honeymoon. Eleven turbulent years and two cherished sons later, they formally separated.

And so, Saturday, the anniversary of the Disneyesque wedding that mesmerized the world was marked only by the calendar.

Even the British tabloids, which reported relentlessly on Diana’s wedding, her subsequent troubles, that icky divorce and her tragic death in 1997 had no major stories.

But make no mistake – Diana’s celebrity is not fading.

It’s just that the focus seems to be on Aug. 31, 2007, which will be the 10th anniversary of the Paris car crash that took her life.

According to the London newspaper the Independent, the royal family is bracing for the release next year of dozens of new tell-all books scrutinizing Diana’s life and death.

(As if the tawdry revelations already published by her riding instructor, her private secretary, her voice coach and her butler were not enough to make us wince from here to eternity.)

CNN’s Larry King is compiling memories for his book, and former New Yorker and Vanity Fair editor Tina Brown was reportedly advanced at least $1 million, perhaps $2 million, for a book about Diana’s impact on the world and her status as an icon.

That’s impressive, says Sara Nelson, editor in chief of Publishers Weekly:

“For a fairly serious person (such as Brown) to be writing a book about a princess nine years later,” Nelson said, “suggests Diana has cultural staying power. There is still a great deal of fascination with her. “

Robert Thompson, a professor of popular culture at Syracuse University, says the absence of 25th-anniversary events is understandable, given the way the couple’s marriage disintegrated.

(Fraught with angst when she discovered her husband was still in love with his old flame, Diana developed bulimia and a penchant for extramarital affairs. Who can forget – better yet, who wants to remember – the audiotape of Charles wishing he were Camilla’s tampon? The tragic tale has been chronicled in more than 2,000 books so far.)

The royal wedding provided an ideal public diversion, “after all the social change of the ’60s and the consciousness-raising of the ’70s,” Thompson says.

“By 1981 most people had come a long distance in their attitudes about women. There was a sense that you could back off from the need to be aggressively on-message” about feminism.

“That’s why this idea of a Cinderella marriage played better in ’81 than it would have in 1975.”

The pomp and ceremony were almost medieval.

Images of Diana’s wedding day show a blushing blonde riding with her father in a glass coach (remarkably similar to Cinderella’s) through streets swarming with 600,000 Londoners.

Her silk taffeta gown had a boned bodice, a bouffant skirt buoyed by petticoats, and a train that stretched for 25 feet – the longest in royal history.

In the 3 1/2 minutes it took for Diana to walk up the red-carpeted aisle of St. Paul’s Cathedral, romantics the world over were riveted to a level of attention that remains taut even today.

The public’s fascination focused on Diana – almost to the exclusion of the rest of the royal family.

And Diana continues to sell, in part because of her remarkable transformation from shy Di to radiant fashionista to emissary for the sick and disadvantaged.

Unlike the rest of the stuffy royals, Diana made herself accessible, likable and, in time, admired.

Her legions of fans cheered when Diana stood up to Queen Elizabeth II, insisting on taking an active role in raising William and Harry. They were anguished when she suffered the indignity of gossip. Diana owned up to her human frailties, and the public admired her grit.

In a way, the fairy tale did continue. The awkward duckling became one of the most beautiful, most photographed women in the world. And to her credit, Diana used her enormous popularity as a bully pulpit for compassion.

She became respected as an advocate for victims of land mines and HIV/AIDS, admired as “the People’s Princess” and “the Queen of Hearts.”

“We like to think her legacy lives on through our work,” says Mitali Atal, spokeswoman for the Diana, Princess of Wales Memorial Fund.