Hey there Michele,
Welcome to planDisney and thanks so much for your question!
The week of Christmas and New Year's Eve are very magical times to visit the the Disneyland Resort - what better way to fully get into the holiday spirit than being in the
Merriest Place on Earth? However, it also
tends to be a very busy time to visit!
There aren't any official Disneyland Resort crowd estimators that project attendance, so I can't point you to any official resources when trying to decide between December 24 and December 26 for your visit. Since this is typically one of the busiest times of year in the parks, it's possible that you wouldn't even feel much of a difference between the days in terms of crowd levels, so it would come down more to which day works out better to visit for you.
If it were me planning a trip for my family and I had flexibility between those two dates, I'd probably pick December 24. While it might end up feeling almost identical in crowd levels, my thought process would be that it would be a fun way to celebrate Christmas Eve with my family!
I also want to note that it's not all "bad news" if you visit during busy periods like the end of December! Often visiting during these times also includes
extended park hours, more entertainment options, and most attractions are operating (as opposed to more being closed for refurbishment during less crowded times in the year).
When visiting during more busy periods, I've found it helpful to take my time, enjoy being at the Happiest Place on Earth, and know that wait times might be longer - especially for the most popular attractions.
I hope that's helpful, Michele! Thank you again for your question and for visiting us here at planDisney.
Have a Magical Day,
Greg