Pippa Middleton's birthday celebrations

The Pippa Middleton Ass Appreciation Society will celebrate her 28th birthday today (06.09.11) with a special event on Friday (09.09.11).

The Pippa Middleton Ass Appreciation Society is planning to celebrate the socialite's 28th birthday.

The sister of Britain's Duchess of Cambridge turns 28 today (06.09.11) and society members plan to mark the occasion via the medium of social networking website Facebook, by celebrating Ass Appreciation Day on September 9.

The founders have jokingly added the event will happen in Prince Harry's bedroom, after reports the pair enjoyed a brief flirtation earlier this year.

The tongue-in-cheek event has been dubbed "a day when Pippa's ass will join the great buttocks hall of fame alongside Jessica Alba and Angelina Jolie".

The British beauty's behind has been revered since her appearance at the nuptials of her sister to Prince William at London's Westminster Abbey on April 29, where she wore a cowl-necked design by Sarah Burton for Alexander McQueen.

Pippa enjoyed a lunch out at a London restaurant yesterday (05.09.11) ahead of her birthday.