Katy Perry doesn't turn the big 2-8 until Thursday, but the partying has already begun! The Wide Awake singer threw a costume party at Los Angeles' Magic Castle on Friday, surrounded by friends to celebrate her big day a bit early.

The most notable guest of the evening? Her on-again beau John Mayer, of course! Even better, the two coordinated costumes, with Katy dressing as a vampire and John as a vampire slayer -- ahhh, love it!! Other important guests at the party include Kristen Stewart (who showed up solo), Neil Patrick Harris (with his partner David Burtka), and Skrillex.

Related Article: Katy Perry's Moving Duet With Autistic Child Jodi DiPiazza

Katy and John dated briefly over the summer, but their relationship was written off as a fling when they broke up in August. However, the two are clearly ready to prove that they're back and better than ever, as they were also spotted together just last week celebrating his birthday!

With Katy still pretty fresh off her divorce from Russell Brand, and John (hopefully) trying to lose his image as a Hollywood Womanizer, could these two just be perfect for each other?! Let us know in the comments section!

Photos Courtesy of: The CObra Snake