Hi, Jennifer!
Great question!
My son and I are headed to Walt Disney World this fall and plan to attend
Mickey's Not-So-Scary-Halloween Party, too; we can hardly wait for all of the festivities!
Should you want to spend an entire day at Magic Kingdom, then you would need to purchase a one-day theme park admission ticket for Magic Kingdom. Typically, the event begins at 7 p.m. and runs through midnight (of course). However, your event ticket for MNSSHP will enable you to enter Magic Kingdom as early as 4 p.m. that day. This is a great way to stick to your budget and skip purchasing an additional park ticket for earlier in the day.
There is a wonderful (unofficial) blog about MNSSHP at AllEars.Net, my favorite, unofficial source of WDW information. Just head to their site and enter "Mickey's Not-So-Scary Halloween Party" into their search bar and click on the September 12, 2012 post by Jack Spence. You'll see tips, photos and additional information that will really give you a great understanding of how to maximize your special event ticket. (For me, this was the third post down on the list of results from my AllEars.Net search.)
Have a wonderful trip, Jennifer! Be sure to send us more questions as you think of them, too; we love to help!