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"Five Alien Elves" is book 3 in the Hamlet Chronicles by Gregory Maquire. Once again, I love the intro to the book so I'm going to quote it to you!
"'Twas the night before Christmas, and you know the rest. The stockings were hung by the chimneys with care and, in one house, Superglue. The youngest kids were already in bed. Many adults were saying, 'If you don't go to sleep, Santa Clause is never going to come!
Something was coming. But it wasn't Santa Claus."
No, instead of Santa Claus there are some kooky aliens from Fixipuddle crash landing near the town of Hamlet! These aliens manage to get everything completely confused, they kidnap Mayor Grass thinking that he's Santa Claus, and therefore an evil overlord. It takes all the kids, Miss Earth, Grandma Earth, and Reebok the dog to save him!