Hi there, Carly, and welcome to planDisney!
Thanks for bringing your question to us and I'm excited for you to visit the Disneyland Resort over the Holiday Season! It's a beautiful time to see all the decorations and
Sleeping Beauty Castle's transformation is absolutely magical!
When you purchase a Disneyland Resort
Theme Park Ticket, adding the
Park Hopper option is an all-or-nothing feature and will cover the entire length of your ticket. It is one flat fee for your entire ticket, so if you're planning on park hopping, it makes sense to get it. You don't have to use this feature at all if you don't want to but it's always nice to know that you have the option. The only way to have Park Hopping for only one day would be to purchase separate tickets for your visit, but I've done the math for this scenario and it's usually more cost effective to purchase Park Hopping for your entire ticket.
The time around Christmas Day can get very crowded, so I would recommend choosing your ticket and securing your reservations earlier rather than later. Reservations are released 180 days in advance, so you'll be able to make yours at the end of June. It might be nice to be able to Park Hop especially if one Park is really busy. You'll be able to just cross the Esplanade to the other Park (check with a Cast Member that it hasn't closed due to capacity but this hasn't happened in a while).
Carly, I wish you the best as you plan your magical Holiday trip! Please feel free to ask us more questions if you need additional help. We're always happy to assist! Have a magical day and we'll see ya real soon!
Wishing You a Great, Big, Beautiful Tomorrow!
-Al