Welcome to planDisney, Jeffrey! I'm so glad to be able to answer your question today.
I totally get wanting to double-check availability before you buy tickets, especially for a busy
holiday week! Here's the easy version: if you're purchasing the standard date-based tickets sold on the Walt Disney World website, you do not need theme park reservations. You'll choose your start date when you purchase, and then on each day of your trip, you'll simply head to the park you want, no separate reservation required.
Reservations still come into play for special admission types, like
Annual Passes and certain promotional or non-dated tickets, such as some
military offers. If you're planning to use one of those, you can check the
theme park reservation availability calendar before you purchase. It shows availability by admission type and date, so you know what's open at a glance.
For your travel window, December 21 through December 24, the calendar is currently showing all four parks available across admission types. If you'll be using a ticket that does require reservations, I recommend purchasing as soon as possible and immediately making those reservations to lock in your preferred days while everything is still wide open. Availability can shift as the holidays approach.
So to sum it all up, with date-based tickets, you're in the clear with no reservations needed, just buy and go. If you're using an admission that does require reservations, the good news is that December 21 through December 24 looks wide open right now, so it's a great time to secure your dates.
Have a magical holiday visit, Jeffrey!
Jessica