On weekday party days (Tuesday, Thursday, & Friday) there is a LOT less people in the parks. Lines are shorter, dining reservations are easier, and you can get so much more done. That's the case for both Halloween and Christmas parties. Sundays are a little busier as more locals tend to visit on the weekend, but nowhere near like traditional Sunday's.
Just be aware that at 4:00 when the party guests are allowed to enter, MK gets mobbed with both day guests and party guests all there at the same time. If you're not staying for the party, 3:30 is a great time to leave, or right after the Festival of Fantasy parade.