Hi, Amanda! Usually the special characters come out during Mickey's Very Merry Christmas Party, so I highly suggest that you purchase the
special event tickets. Last year Jack Skellington was out as "Sandy Claws". The Seven Dwarfs, Captain Jack Sparrow, Rapunzel and Flynn Rider were all out along with many others. You can also meet Santa Claus next to City Hall or in Epcot. Details for this year still haven't been released, so keep checking back with the Disney Parks Blog for any new information. If you plan on only visiting with regular park admission they do have several characters in holiday clothing throughout the parks.
I do definitely suggest attending Mickey's Very Merry Christmas Party, though. It is actually my favorite thing to do at the parks during the holiday season. Not only do you get to meet some pretty cool special characters, but it also snows on Main Street U. S. A. and the Once Upon A Christmastime Parade is really fun. Last year during the event the attraction wait times rarely went over 20 minutes which was amazing. I walked on to a lot of popular attractions and rode Splash Mountain multiple times that night.
I hope you consider going to Mickey's Very Merry Christmas Party, and if you need anything else you know where to find us. Have a magical holiday vacation!