Britney Spears celebrated her birthday in an unusually sedate and low-key fashion on Saturday (01Dec07) night. Spears, who turned 26 on Sunday (02Dec07) and is known for her wild partying, was joined by socialite Paris Hilton, friend Sam Lufti and her cousin Ali Sims for the surprisingly uneventful bash at the Scandinavian Style Mansion in Bel Air, Los Angeles. The singer, who sported long, blonde hair extensions, reportedly blew out candles on a small round chocolate cake while her guests gathered to sing Happy Birthday. A party-goer tells OK! magazine, "She was smiling and looked really happy." The group continued the party at the Four Seasons hotel in Beverly Hills, where Hilton was spotted leaving just before 4am. Spears is said to have spent her actual birthday with her two sons, two-year-old Sean Preston and Jayden James, 14 months.