Hi Brian!
Welcome to planDisney.
The simple answer is "yes"! All guests can enter Magic Kingdom from opening on special party days, but if you don't have a ticket for
Mickey's Very Merry Christmas Party, you must 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 and continues until midnight.
Generally the park can 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 lower 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). You may find that you enjoy more attractions than usual, with the park possibly being quieter, until 6.00pm.
Every guest can make dining reservations at restaurants inside Magic Kingdom Park up until 5.55pm - if available - (for Cinderella's Royal Table or
Be Our Guest Restaurant, for example), so you could stay longer for that reason, but please know that as soon as you have finished your meal you will be escorted from the park - with no time for 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