Hi Amanda,
According to the Park Hours and Park Schedule on the Walt Disney World website, on December 16, 2016, the
Magic Kingdom Park is scheduled to be open from 9:00 AM - 7:00 PM (with Extra Magic Hours available 8:00 AM - 9:00 AM).
Mickey's Very Merry Christmas Party will be held from 7:00 PM - 12:00 AM and as you quite rightly say, separate admission is required for this special event.
On December 20, 2016, the Magic Kingdom Park is scheduled to open from 9:00 AM - 10:00 PM (with Extra Magic Hours available 8:00 AM - 9:00 AM) with
Wishes nighttime spectacular due to take place at 10:00 PM.
Disney's Festival of Fantasy Parade will be taking place at 3:00 PM on both December 16, 2016 and December 20, 2016.
Should you decide to purchase Special Event tickets to Mickey's Very Merry Christmas Party, you would be able to see the holiday version of Wishes nighttime spectacular called Holiday Wishes as well as Mickey's Once Upon A Christmastime Parade which will not be available at the Magic Kingdom Park on December 20, 2016.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma