Katy Perry got into the Halloween spirit this weekend -- by celebrating her birthday with a vampire-themed bash!

The singer turned to the dark side on Friday night, donning fangs, white makeup and some creepy red contact lenses to her party at the Magic Castle.

Looking sexy in a white lace dress, Perry shared some PDA with on-again boyfriend John Mayer -- who wore some serious facial prosthetics for his zombie hunter getup.

Other guests included Kristen Stewart, Neil Patrick Harris, Shannon Woodward, Skrillex, songwriter Diana Warren and stylist Johnny Wujek.

The party looked like an absolute blast -- check out more photos over at TheCobraSnake.com. They'll make you very jealous.

Katy celebrates her 28th birthday on Thursday.

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