We are doing to WDW 12/10-12/16. Because some of our party is coming in a couple days late and we are going to MVMCP on Sunday, I am left with Saturday or Monday to focus on priority rides for MK. The crowd calendar lists Saturday as a 2 and Monday as a 3. Saturday has EMH late which we will not use. When I look back over the last three years in the crowd calendar, one year lists them as both 5s, and then then the other two years Saturday was higher. Could there be a reason that Saturday is listed as lower despite previous years? I am tempted to switch my MK day to Monday despite the calendar suggesting Saturday is better. I know both 2 or 3 are excellent days for touring regardless. However we have one child with autism, one with ADHD, and a senior riding on a scooter (first visit to WDW for all of those) so we are trying to get the best possible crowds to help our special needs group. I haven’t been in 25 years so any tips and suggestions welcome.
I don’t think there will be a huge difference between the days. Are they doing all the same parades and shows both days? That can sometimes make a difference. They don’t always to the Main Street Electrical Parade every day, so if you go on a day when it is NOT happening then the crowds will be lower. Lots of people choose to go on parade days because they don’t want to miss seeing it.
The difference between a 2 and 3 should hardly be noticeable to you. I would personally go to MK on Saturday before the party. Then you can re-ride anything you particularly enjoyed at the party. I know that my children would have a hard time enjoying the special activities of the party with so many rides unridden. Just something else to consider.
@crazy has a good point about the little ones wanting to ride and getting distracted from the party activities. Your party day is Sunday and I would tour MK Saturday, then arrive at 4 for the party and enjoy some low waits to ride things you missed or want to double up on. Enjoy!
Thanks for all the good posts! You guys convinced me to stick with our Saturday MK day. Having shows the second day after riding the day before is a great tip. I guess I was nervous that a 2 on a MK weekend was too good to be true ! 
We are so excited - thanks for the help.