Singer Katy Perry turns 28 on October 25 but she already had an early birthday celebration during the weekend with a Halloween-themed costume party at "The Magic Castle" in Los Angeles.

At the birthday bash, Katy Perry reportedly had a couple of celebrity guests which included her on-and-off-boyfriend John Mayer, Kristen Stewart, songwriter Diane Warren, Neil Patrick Harris and "America's Next Top Model" Johnny Wujek. Instead of receiving gifts, the "Part of Me" singer requested donations in order to support the David Lynch Foundation for Transcendental Meditation.

At the birthday bash, Katy Perry dressed up as a sexy vampire while 35-year-old John Mayer managed to look dashing with his vampire hunter ensemble. Usmagazine.com reports John Mayer absolutely had Katy Perry's full attention during the celebration. "They were in their own world. She didn't even talk to her friends much. It was just her and John. She's obsessed with him. She said hello to people but then went right back to him," a partygoer revealed to Us Weekly.

The past year has been tough on Katy Perry after having gone through a divorce from British husband Russell Brand back in December 2001. After the divorce has been finalized, Katy Perry started dating John Mayer. Their relationship has always been in the center of controversy with numerous break-up reports which had her family and friends concerned.

In the past, the Grammy Award-winning music artist has already wooed several famous Hollywood ladies like Jennifer Aniston, Jessica Simpson, Cameron Diaz and Taylor Swift. However, Katy Perry believes that she will be able to change John Mayer's notorious womanizing ways. According to a source telling Us Weekly, Katy Perry "doesn't want to be just another one of his conquests."

Katy Perry poses at the premiere of "Katy Perry: Part of Me" at the Grauman's Chinese theatre in Hollywood, California June 26, 2012.

Katy Perry

John Mayer

Pop star Katy Perry wore a slime-green bra and purple hair at the Kids' Choice Awards 2012 show in Los Angeles on Saturday.

Pop star Katy Perry was voted in at number four

Pop sensation Katy Perry and British comedian Russell Brand

Singer Katy Perry poses on arrival at the 53rd annual Grammy Awards in Los Angeles, California February 13, 2011.

Singer Katy poses for photographers after winning Video of the Year for "Firework", Best Collaboration, and Best Special Effects at the 2011 MTV Video Music Awards in Los Angeles August 28, 2011.