Mariah Carey Wears 'Alvin & The Chipmunks' Costume at Horror Nights

Mariah Carey must've watched "The Nightmare Before Christmas" recently ... 'cause she's blending Christmas and Halloween perfectly at Universal's Halloween Horror Nights!

The singer-songwriter hit the annual Halloween bash earlier this week, outfitted in an "Alvin & The Chipmunks" costume ... pretty hilarious considering the little rodents have almost as much Christmas clout as MC.

In the vid, Mariah's milling around The Wizarding World of Harry Potter ... with enough security to protect from any vampires or werewolves who might want to take down the Queen of Christmas.

BTW ... we're told her kids were with her at the event -- but, they don't appear to be in the vid. Unclear if they were grabbing a Butterbeer, but sounds like the whole crew enjoyed an evening together.

As we've told you ... Mariah's already changed the calendar on IG -- going from Morticia Adams tangoing with Gomez to her normal Mariah Claus ensemble for the Christmas season in a clip she posted minutes after November 1 hit.

Mariah's got an even more chaotic Christmas season than usual BTW. Next week, she begins a string of 21 shows that will end just before Christmas -- with appearances in Hollywood, Dallas, Atlanta, Boston, Baltimore, Philadelphia, Brooklyn and more scheduled.