Britney Spears It's Birthday Britney, Bitch

Britney Spears celebrated her 35th birthday with roughly 20 thousand people Friday night, and revamped an old 'do for the occasion.

Spears performed at KIIS FM's Jingle Ball in L.A. -- along with Justin Bieber, Bruno Mars and a bunch of other huge acts -- and her double bun hairstyle made loyal fans nostalgic for that classic, often-imitated, never duplicated "school girl in pigtails" look.

Little messier these days, but still ... cue the track!