Hi Rosemarie,
Mickey's Not-So-Scary Halloween Party is a wonderful event! I have been to this party and had such a great time. From trick-or-treating around Magic Kingdom, to special fireworks, parades and entertainment, it is so much fun. I truly recommend going if you have the chance.
Yes, Magic Kingdom closes early on the nights of Mickey's Not-So-Scary Halloween Party. It typically closes to day guests at 7:00 p.m. on these party nights. I suggest that you take a look at the
Walt Disney World park hours calendar to see the hours for the dates that you plan to be at Walt Disney World. If there is a day that you are not attending Mickey's Not-So-Scary Halloween Party, but there is one planned for that evening, which will be indicated by a 7:00 p.m. closing time for Magic Kingdom, consider how you plan that day. You might want to consider either spending the day at another theme park rather than Magic Kingdom if you do not have the Park Hopper option, or if you do have the Park Hopper option, taking advantage of that to hop to another park for the evening. Yes, you do need to purchase a ticket especially for the party if you plan to attend.
I hope that these thoughts are helpful for you, Rosemarie! Have a wonderful time planning your magical vacation, and please do let us know if you have any further questions.