Hi Heather!
Mickey's Not So Scary Halloween Party is absolutely spectacular, or as Disney likes to say, SPOOKtacular!
Generally, Guests with passes to this event are usually allowed to enter the Magic Kingdom at 4:00 p.m. The event does not officially begin until 7:00 p.m.
If you arrive early, before 7:00 p.m., all of the attractions that are open for that day will still be open. However, after 7:00 p.m. there were only be certain attractions that will still be in operation. For a look at the event guide, click here:
MNSSHP Event Guide. It will list which attractions are operational, as well as times and locations for special events such as Mickey's "Boo To You" Halloween Parade, Hallowishes (fireworks), character dance parties, and more.
And, yes, the "My First Visit" buttons are complimentary, and can be picked up at Disney Vacation Planning sites or Guest Services. You may also inquire at the Front Desk at your Disney Resort if you are staying on Disney Resort property.
Have a wonderful time at this special event!