Hi 18!
Since Mickey's Very Merry Christmas Party starts at 7pm, I would at least try to make it to Magic Kingdom by 6:30pm, just to get through the security checks and entrance gate. If you have tickets to the Party, that's all that's required to enter the Park. In terms of must-see? You got to see all of it! :-)
--Actually, the two "can't-misses" are Mickey's Once Upon a Christmastime Parade, and Holiday Wishes. The parade in particular, which I think fills most of the Disney characters I can think of. Then it builds to a grand finale with Mickey, Minnie, and...well, it would be a spoiler, so you just have to wait and see...
If this event doesn't get you into the spirit of Christmas, nothing will! :-)
Have a Mickey Merry Christmas!