Hi Justin,
You can absolutely visit the Magic Kingdom Park on Halloween (October 31,2016).
Regular park admission is required by all Guests wishing to visit the theme park during it's regular park hours that day (which are currently scheduled as 9:00 AM - 7:00 PM on the Walt Disney World website).
From 7:00 PM to midnight,
Mickey's Not-So-Scary Halloween Party will be taking place for which a Special Event Ticket is required.
All Guests who have been at the Magic Kingdom during the regular park hours that day will be required to leave the theme park at 7:00 PM unless they have Special Event Tickets for Mickey's Not-So-Scary Halloween Party.
Should you want to attend Mickey's Not-So-Scary Halloween party on October 31, 2016, I would highly recommend purchasing Special Event Tickets as soon as possible. The parties are super popular and Halloween night in particular will almost definitely sell out ahead of time.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma