Hey there, Xiaoqing!
Thanks for bringing your question to the Disney Parks Moms Panel. I'd be delighted to help with your
Disneyland Resort ticket question.
I'm happy to let you know that you can use your ticket in the parks on the day it expires, which in your case is on December 21, 2019. Starting December 22, 2019 your ticket will be invalid.
You are going to be visiting during the
most magical time of the year as holiday decor, attractions, Characters, and treats are all over both Disneyland Park and Disney California Adventure Park. You won't want to miss seasonal favorites like "it's a small world" Holiday, Haunted Mansion Holiday, Mater's Jingle Jamboree, and Luigi's Joy to the Whirl. Also, make sure to arrive on an empty stomach so you can enjoy holiday treats galore as well as comfort food from around the globe at Disney Festival of Holidays at Disney California Adventure Park.
Please return if you have any other questions, and have a holly jolly day!
-Jennifer