Hi Matthew!
A big congratulations on taking your 1st Walt Disney World vacation,in just a few short weeks you will be on your way!Mickey's Not So Scary Halloween Party starts at 7 pm,so you do not have to use park admission unless you will be visiting a park earlier in the day.While you can't "cancel" your park admission, you could lower the amount of days on your tickets.My family will sometimes spend the day sleeping in late and enjoying the pool on Mickey's Not So Scary Halloween Party nights,in order to get ready for the big event.
The first place I usually start when making my itinerary is by deciding on which parks or special events I will be visiting for everyday of my trip.You didn't say what day you will be attending Mickey's Not So Scary Halloween Party so I will just make a notation for which days it will be offered.Plus on your arrival and departure days I will include the best parks to visit in case you have time.Typically, after deciding which parks I will be visiting I start making dining reservations for table-service locations or noting which counter-service locations I would like to visit.Based on your travel dates, here is my suggestion for your itinerary:
9/21 Epcot 9am-9pm
9/22 Hollywood Studios 9am-8pm
9/23 Magic Kingdom 9am-10pm
9/24 Animal Kingdom 9am-6pm **Mickey's Not So Scary Halloween Party at Magic Kingdom**
9/25 Magic Kingdom 9am-10pm (or Animal Kingdom if you choose to not visit the day of Mickey's Not So Scary Halloween Party)
9/26 Epcot 9am-9pm
9/27 Magic Kingdom 9am-7pm **MIckey's Not So Scary Halloween Party at Magic Kingdom**
One thing I cannot stress enough, use FastPass!Disney's handy invention will keep you from waiting in lines, especially on popular attractions.I hope this was helpful and please visit me again if you need anymore advice for your upcoming trip.
Happy Planning!
Mandy