Hi Ashley,
December is a fantastic month to visit Disney World! You have timed it perfectly, as you will get to enjoy all the various holiday events, shows and decorations, and be leaving just about the time the Christmas crowds arrive.
Cinderella Castle is breath-taking when all lit up in its holiday lights. Yes, you will absolutely be able to enjoy the Castle in its amazing holiday glory without attending Mickey's Very Merry Christmas Party. The Party is offered on select evenings throughout November and December, however Cinderella Castle is lit nightly whether or not there is a party occurring. The scheduled "lighting ceremony" will be listed on the park hour calendar for Magic Kingdom, but even on the evenings of a Christmas Party, the Castle is lit before the park is closed to party attendees.
On a side note, if you have the open evening, you may want to consider purchasing tickets for
Mickey's Very Merry Christmas Party. This special ticket event is open to a limited number of tickets which means the crowds are usually much smaller and wait times are typically minimal. There are a number of special events and shows offered on party nights only as well as a special holiday parade and fireworks show. We love the extra time to visit with the Disney pals in the winter attire, and there is something about snow on Main Street while enjoying fireworks over Cinderella Castle that warms even the biggest scrooge's heart.
Wishing you a magical time!