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!