Katy Perry
turns 28 on Thursday, and she kicked off her birthday festivities by throwing a costume party Friday night in Hollywood!

So which famous pals turned up for the bash? On-off boyfriend John Mayer-- who was dressed as a zombie-- was in attendance and was by Katy's side all night. "They looked like they were having a blast together," a source tells Cambio. "There was lots of PDA. They were kissing and dancing all night. It was really cute!"

Perry opted to dress up as a sexy vampire. Could that have been inspired by another vampire in attendance? Twilight beauty Kristen Stewart hit the party at LA's famed Magic Castle and appeared to be in "great spirits" a source tells us.

Dressed in a white tee, jeans, jacket and glasses, Stewart was spotted laughing and mingling with friends. Noticeably absent was back-on boyfriend Robert Pattinson.

But don't worry, these two haven't split again! Rob was in Sydney over the weekend to promote Breaking Dawn-- Part 2. Kristen Stewart has also started her press duties kicking things off in Tokyo Monday.

Rob and Kristen will reunite next week in LA ahead of the Breaking Dawn premiere where they are set to make their first public appearance since the Rupert Sanders cheating scandal. "Things were obviously tough at first, but they are back together and in a weird way seem stronger than ever," sources tell Cambio of the couple.

Glad these two were able to sort things out!
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