The whole birthday concept seems to be a tough one for Justin Bieber to grasp — at least the part about it being one day, that is. Bieber, who actually turned 21 on March 1, followed up the recent taping of his Comedy Central roast with yet another birthday party, this time in Las Vegas. "He was in great spirits," a source tells E! News. "He was having a great time with his friends. He looked good." Oh, good. The club that threw the lavish bash even gave him a cake shaped like the Lincoln Continental from "Entourage," and are you really surprised he's an "Entourage" fan? I mean, come on. Now I'm just curious what his upcoming two-month-long Memorial Day celebration will look like. 

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