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Khloe Kardashian, Instagram

Instagram

Hey Khloe Kardashian: This is a birthday party, not a costume party!

The Keeping Up With the Kardashians star joined Kourtney Kardashian, Kendall JennerKylie Jenner and other family members to celebrate Mason Disick's birthday Saturday afternoon.

While some members of the family appeared to be dressed casual, Khloé went all out to celebrate her nephew's special party.

"Kourtney pulled a ‘mean girl' move on me and had me dress like Sally from The Nightmare Before Christmas while everyone else was mad chill," she wrote on Instagram while posing with red hair and a full face of makeup. "lol thanks @kourtneykardash #MasonsBirthdayBash"

Not to be outdone, makeup artist Joyce Bonelli also decked out the little ones including Mason and his friends. "Killing it with Jack and Sally Skellington," she shared online. "#MASONSBDAYPARTY @KhloeKardashian @KendallJenner."

Mason Disick, Instagram

Instagram

Some may be wondering why Mason is having a birthday party so early. After all, he doesn't turn five until Dec. 14.

From what we've seen of Kourtney on her E! shows, the busy mom likes to plan. With a baby on the way and the holidays quickly approaching, why not party earlier than later?

In fact, an inside shot of the party taken by Bonelli shows a giant statue of Jack Skellington next to a completely decked out Christmas tree. Hey Kris Jenner, has the decorating already begun too?

What we do know is Mason appears to be going through quite the spooky face. 

Mason Disick, Instagram

Instagram

Weeks before Halloween, Kourtney treated her son to a trip to Disneyland where they had to stop at the Haunted Mansion ride. And while he dressed as a full-on Ninja for his visit to the Happiest Place on Earth, Mason ultimately went as Darth Vader on Halloween night.

"My mom got him a Cat in the Hat that he was so mad at. He threw it in the trash and poured sparking water over it," Kourtney shared with E! News. "He was so offended by Cat in the Hat."

See, this birthday party had a smart theme after all!

To see more of Mason, watch Kourtney & Khloé Take the Hamptons Sunday nights at 9 p.m. only on E! 

Mason Disick, Instagram

Instagram

Mason Disick, Instagram

Instagram