Britney Spears waited a week to go wild for her birthday -- the Queen B celebrated Friday night as she wrapped her Femme Fatale tour in Puerto Rico. She Tweeted a photo of her blowing out the candles on her cake.

It was the big 3-0 for Brit a week ago and reports claim Spears' boyfriend Jason Trawick, 39, is prepping to pop the question before the end of the year. Or maybe on New Year's Eve? ... We can't wait to see the ring!

Oh, and check out another pic Brit Tweeted ... her assistant Brett surprised Britney on stage during Big Fat Bass. Cute!