Hi Elizabeth,
Congratulations on your upcoming Christmas celebration at Walt Disney World! I think you have picked a fabulous time to visit since historically the crowd levels dip down after Thanksgiving weekend until the week prior to Christmas. Overall your weather should be mild, your crowd levels moderate, and the holiday cheer in full swing!
In the past Mickey's Very Merry Christmas Parties tend to have lower attendance Monday through Thursdays. Weekend parties draw fans from the local area which tends to increase crowds somewhat. With the party dates you have available to you (Sunday, Tuesday, and Thursday) I would personally choose Tuesday because it is in the middle of the week. However you should experience mild to moderate crowd levels with any of your available choices. As long as you have entered the park prior to 7:00 PM and have a plan for what you want to do you will have a Magical experience.
Here is a link to view and purchase
Mickey's Very Merry Christmas Party tickets for 2015. If you arrive after 4:00 PM you may use your party ticket to be admitted to Magic Kingdom. That's 3 hours of bonus Magic Kingdom time prior to the party getting under way! However if you are already visiting Magic Kingdom on your party date you do not have to exit and re-enter. In the late afternoon there will be stations set up throughout Magic Kingdom where you may have your party tickets scanned and receive wristbands. Look for signs that say something like "Event Wristband Distribution" but if not readily apparent to you, simply ask a Cast Member where the closest wristband distribution is located.
Thank you for your question and welcome to your Disney Home for the Holidays!
Brenda ºOº