Julie:
Thanks for coming to visit us here at the Mom's Panel.
The week between Christmas and New Year's is always the busiest time of the year at Disney World, so I would definitely eliminate New Year's Eve, even though the parks are open until the wee hours and you can ride a lot of attractions during this time. Although it's unique to say you've been to Disney World on New Year's Eve, I don't recommend it for first-timers.
New Year's Day takes a dip usually when it comes to crowds during that week because, well, folks stay up REALLY late to take Disney up on those late Extra Magic Hours on New Year's Eve but making that day work requires you to get to the parks early after your own New Year's Eve celebration. My recommendation is to definitely go to the parks on Saturday, December 20th. Those coming for Christmas will probably not be arriving until Sunday or Monday, so your overall crowd factor won't be as intimidating. Additionally, local Florida residents will probably still be focused on last-minute, weekend Christmas shopping, minimizing the local factor. With your park hopper ticket, I would recommend that you not start your day at the Magic Kingdom but end it there as they are
scheduled to stay open for regular visitors until 1:00 a.m. and have Extra Magic Hours from 1 a.m. to 3 a.m. for resort guests.
Wishing y'all a magical vacation!
Whitney