Pippa is thought to be throwing her sister a surprise 80s-themed party in London. Where's our invite? Photo: Getty
"The party has been months in the planning. Pippa and Harry have been in the thick of it, and have recruited a bunch of friends to help," a royal inside source told the magazine.
"The guests have been sent 'save the date' cards, and told that it will probably be in London.
"Given the people organising it, you can be sure it will be brilliant."
According to Grazia, Kate has also requested that guests make charity donations in her name instead of bringing birthday gifts - although reports suggest that Prince William has had an oil painting commissioned to mark the milestone.
It's widely expected that the Duchess will be walking the red carpet with her husband at the London premiere of Stephen Spielberg's War Horse on Sunday night, with the party taking place afterwards.
Here's hoping Kate doesn't read the press. Otherwise this party won't be much of a surprise...
See our pick of the birthday girl's perfect 10 style moments:
- November 2011<p> Duchess of Cambridge wore a Jenny Packham silver grecian dress to a dinner reception in aid of the National Memorial Arboretum Appeal.</p>

- September 2011<p> </p> <p> The Duchess arrived to open the new Oak Centre for Children and Young People at Royal Marsden Hospital, wearing a simple oatmeal, felt long-sleeved Amanda Wakeley dress.</p>

- August 2011<p> </p> <p> Duchess Kate stepped out at Summerfield Community Centre in an Alexander Mcqueen silk blouse and pencil skirt.</p>

- July 2011<p> The Duchess of Cambridge wore a printed coat dress for the wedding of Zara Phillips and Mike Tindall.</p>

- July 2011<p> The Duchess of Cambridge departs her visit to an inner city school in Los Angeles in her Whistles outfit.</p>

- July 2011<p> Catherine, Duchess of Cambridge wore a lilac Alexander Mcqueen gown to the 2011 BAFTA Brits.</p>

- July 2011<p> The Duchess of Cambridge at Santa Barbara Polo Club in California.</p>

- July 2011<p> The Duchess of Cambridge sizzled in scarlett at a reception at Calgary Zoo in Canada.</p>

- July 2011<p> </p> <p> Kate wore a patterned white shirt, flared jeans and classic brown cowboy boots to the Calgary Stampede parade in Canada.</p>

- July 2011<p> The Duchess of Cambridge visits Slave Lake in a khaki Burberry shirt and her favourite J brand jeans.</p>

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