Hi Ian!
Welcome to planDisney.
You are correct! All guests can visit Magic Kingdom on special party days, but if you don't have a ticket for
Mickey's Not-So-Scary Halloween Party you will be required to leave by 6.00pm. Party guests can enter Magic Kingdom any time after 4.00pm, even if they don't have a regular admission ticket for the park, and will be identified by a special party wristband. The party officially begins at 7.00pm.
The park is reported to be quieter during the day, as some guests would rather save their ticket for a full day when they can stay until closing, and enjoy the fireworks. If you don't mind having a shorter day you might enjoy reduced wait times around the park, and you can still make your
Lightning Lane purchases if you wish to do so (but you might not need them).
All guests are permitted to make dining reservations up until 5.55pm - if available - (for Cinderella's Royal Table or
Be Our Guest Restaurant, for example), but as soon as you have finished your meal you will be escorted from the park - no shopping, or stopping for photos.
Please come back and visit us if you have any further questions, and you can
send me a direct question if you wish. We are always happy to help you plan your magical vacation to Walt Disney World!
Sue