Britney Spears celebrated her 26th birthday with friends Sam Lutfi and Alli Sims at the opening of the Scandinavian Style Mansion in Bel Air on December 1st. Although her official birthday is on the 2nd, that didn't stop the pop princess from having a terrific time. Britney celebrated with a chocolate cake; probably her favorite from the silly look on her face? Britney wore a skin tight black spaghetti strap dress that was so short her purple panties peeped out! Spears and the gang seemed to have a fabulous time before heading out to the Four Seasons Hotel with Paris Hilton.