Blake Lively’s birthday party is going to be a celebration worthy of her extravagant Gossip Girl alter ego. The actress will roar in her 21st year with a Jazz Age-themed bash on September 6 in New York. “All the guests have to wear 1920s-style clothing,” says an insider, adding that Blake has some demanding guidelines. “The doormen are checking and will reject those not in theme.” Blake — who turned 21 on August 25 — also celebrated her big day with cake on the set of her show.

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