Hello Borja!
Welcome to planDisney! December in Walt Disney World Resort is a
wonderful time to visit. There are two special ticketed Holiday events going on when you arrive and I strongly suggest you check them out. The first is the classic
Mickey's Very Merry Christmas Party at Magic Kingdom Park. I love seeing Cinderella Castle covered in snow and the characters in their holiday best. This year we have a brand-new party at Disney's Hollywood Studios called
Disney Jollywood Nights. More details are going to be coming soon and posted on the
Disney Parks Blog but right now it's looking to be a glamourous holiday evening. If you have one day to spend in the Parks I would try to time it to include this special event or just go to the special holiday party. You are still going to a Park and able to ride attractions but it becomes even more magical! It's great if you are a night owl and have the stamina to stay up late.
I visited both the
Walt Disney World site for Spain and the one for the
European Union and it appears that you are only able to buy 2-day tickets or greater at this time. Unfortunately, that is the product offered to our friends in the European Union. The only option that I can suggest is that you can buy your Park tickets when you arrive in the United States. You can as soon as you land, visit the
American Walt Disney World site and purchase a one-day ticket. Since you are only looking to buy a 1-day ticket, you will select your Park entry day during the purchase process and will not need to make another
theme park reservation. That will be one less worry for you as this all be a last-minute situation. You can, however, still book
dining reservations 60 days before your anticipated arrival even without Park Tickets in place so you should think about that now.
I'm sorry that I don't have the answer you might have been looking for but I hope it helps.
Please let us know if you have any more questions as we are glad to help!
Happy Planning!
-Nimmy