Hiya, Shara!
Welcome back to planDisney. Thank you so much for your service and for stopping by with your question.  How exciting that you plan to visit Walt Disney World Resort on the merriest day of the year.  Christmas and Disney go hand in hand, in my opinion, and I love seeing the parks and characters decked to the halls in their holiday best!  It is the jolliest time indeed.
Shara, typically, date-based tickets do not require a theme park reservation; however, some military tickets do require a theme park reservation.  Additionally, some military tickets have block-out dates. 
You can check if your tickets need a theme park reservation by logging into your My Disney Experience account and then following these steps: 
 - First, make sure your theme park tickets are linked to your account. 
 - Then, click on the
 theme park reservations page.  - Click on the blue button that says "Make or Modify Reservations." 
 - Next, click on the box that says "Guests With Tickets or Passes" and click "Make a Reservation." 
If your tickets do not require a reservation, it will tell you that you don't need one, and if you do require one, it will allow you to make reservations for any of the available theme parks.   I always recommend doing this, just to make sure you have all of your Donald Ducks in a row before you arrive.  And, I would do this soon, especially when visiting during the holidays, as reservations can get booked up.  
On that note, Shara, I would also recommend arriving to Magic Kingdom as early in the day as possible.  Christmas Day can get quite busy, as many Guests love to visit the parks on this day, and sometimes the theme parks can reach capacity.  While Mickey and friends can estimate the number of Guests who will be visiting, he does sometimes close the gates if it is too full.  
But, if you plan ahead with 
Advance Dining Reservations, Lightning Lane Multi Passes and Single Lane Passes, arriving early, utilizing quick service dining (especially pre-ordering in the My Disney Experience app), and taking advantage of the immersive magic of the shows and parades, you will have a magical time. Remember, Disney is an experience, not just a destination, and pixie dust is everywhere.  This is how we have the merriest vacation when we visit Walt Disney World during the holidays.
I hope this helps you, Shara! If you have any further Disney vacation planning questions, soar back here to see me.  I'll be right here, at the second star to the right and straight on 'til morning.
See ya real soon!
Keri