What will Kate do on her 30th birthday? Kate, Duchess of Cambridge, arrives for a Gary Barlow concert in support of The Prince's Trust on Dec. 6, 2011, in London. (CBS) The Duchess of Cambridge will hit a major milestone on Monday - her 30th birthday - and there are conflicting reports about how she is planning to celebrate. British magazine Grazia reports that Kate's sister, Pippa Middleton, is planning an '80s-themed party along with Prince Harry. There will be karaoke and cocktails, the magazine adds, and a guest list that includes the Beckhams. Pictures: Kate, Duchess of Cambridge However, the U.K.'s Metro reports, citing a St. James' Palace spokesperson, that the reports of a big bash are untrue. The rep refused to give details on Kate's birthday plans, but said, "Whatever is planned will be low-key and private." Whether she has a blowout or a quieter celebration, Kate's birthday weekend is also said to include a film premiere. According to British reports, she and husband Prince William will attend the London premiere of the film "War Horse" on Sunday, the eve of the duchess' actual birthday. The Daily Mail adds that the pair will head to a charity dinner after the event.