Hi Hyunwoo Paul! Thanks for bringing your question to planDisney.
Your trip is almost here! I bet you're excited, and you should get to see a lot of the magic of the
Holidays at the Disneyland Resort. We'll be visiting during this festive season as well. I'm ready to go with my must-see list and must-eat list. Check out
Disney Eats, the official food and beverage news source for the parks, for information about what jolly offerings you can expect in the parks during your visit.
When you purchase your
tickets from Disneyland Resort directly, you're asked to select a day and a park to start your day when you choose the Park Hopper option. The calendar selects the appropriate tier and price and once you check out, your preferred park and date becomes your theme park reservation. If you already have your ticket, I suspect you've already got a reservation for your park day, which is why you can't select another one. I hope that's good news.
You can search your e-mail for a theme park reservation confirmation or look for your confirmed reservation on the Disneyland Resort website under the "My Disneyland" dropdown menu. Alternatively, the Disneyland mobile app, which is a useful tool once you arrive, will show your reservations under "Future Plans," which is located under the three-bar menu on the bottom right of the app. Suppose, however, you purchased your tickets through a travel agency. In that case, I suggest contacting them to confirm whether you have a theme park reservation for your preferred park and date or whether they can assist you in making one.
I hope this helps!
Elizabeth