And you thought Blair threw some lavish parties! Blake Lively is planning a huge Roaring 20s-themed shindig for her 21st birthday. "All the guests have to wear 1920s-style clothing," an insider tells In Touch. "The doormen are checking and will reject those not in theme." Better get your flapper dress and pearl strands together quickly -- the party is tomorrow! Click here to see the invite. But don't even think of crashing -- there's no address on it.