How does a princess celebrate the start of her third decade?

Today is the day the world will find out as  Kate Middleton, the newlywed bride of Prince William, turns the big 3-0.

The Duchess of Cambridge spent her birthday eve in the public eye, attending the London premiere of the Steven Spielberg film "War Horse," in Leicester Square, followed by a charity dinner.

Wearing an Alice Temperley-designed black, lace gown, Middletown stole the red carpet, as William dutifully protected her from the rain by holding an umbrella.

Even the movie's A-list director, Steven Spielberg, seemed in awe of the Duchess.

"I hope I'll have the opportunity to wish her in person a happy birthday," he told reporters on the red carpet.

Exactly how the royal couple will celebrate Kate's actual birthday today remains to be seen, and speculation around the milestone has become a parlor game in England, and even here in the U.S.

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The UK's Daily Mail got royal watchers buzzing with a Jan. 5 article speculating that Kate's younger sister, Pippa Middleton, and royal brother-in-law, Prince Harry, were planning a blowout, '80s themed party at a "secret pop-up venue" in London.

The article said the party would feature cocktails and karaoke, and that guests, said to be close family and friends, had already received their invitations for the "secretly arranged party."

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The Daily Mail's report came from Grazia, a British fashion magazine, which cited a "royal source" in saying the party had been "months in the planning," and that Pippa and Harry have "been in the thick" of the planning.

Those plans did not seem so far-fetched, considering 26-year-old Prince Harry is known to enjoy a good time, and Pippa Middleton is somewhat of a professional party planner herself.

The 28-year-old sister of Kate threw a New Year's Eve party at the Middleton family's Berkshire home attended by Prince William and Kate. The event for 100 guests was said to include a $5,000 teepee-style tent, known as a kata, in the backyard, rented just for the New Year's Eve affair.

The siblings from a family of party planners, (parents Michael and Carole Middleton own the U.K. party supply company Party Pieces), and Pippa has signed on to author a party planning book to be released this year.

But, the story was not so, said the palace, which issued a brief statement on the couple's plans, or lack thereof, to celebrate Kate's special day.

"Whatever is planned will be low-key and private," said a spokesman for St. James's Palace.

In addition to the party guessing game, royal watchers have also had fun speculating what kind of gift befits a princess turning 30.

Some reports claim that William is commissioning an oil portrait of his bride, to commemorate their first year as royal newlyweds together, and shopping for a more thoughtful gift, like a watch.  Others have Prince William's grandmother, Queen Elizabeth, giving the Duchess a tiara from the royal collection.

One story the media may have correct in all the Middleton mania is her wish that family and friends make a donation to charity in lieu of buying her a gift.

Middleton drew further comparisons to her late mother-in-law, Princess Diana of Wales, when she  announced this week the array of charitable organizations to which she would donate her time and attention.

Among the groups who may benefit from Kate's turning 30 on Monday are a group of children's hospices, an art therapy and addiction charity, a world-famous art gallery and the Scout Association, the British equivalent of the Girl Scouts.