Hi Julia!
The Christmas and New Year holiday time is definitely one of the most crowded times for a visit to the Disneyland and California Adventure parks. This is the case at all Disney Parks around the world as guests have vacation time off and the holidays are very special at Disney so it draws more and more each year.
It is not a good time to go when you wish to ride all of the attractions. Lines will be long for even the most basic ride or show. So if that is your main reason for a visit then this is not a good time to head there. However, I enjoy a park visit in even the most crowded days to just walk and see the holiday decorations. Hearing the music and seeing the happy faces makes me glad for a visit.
If your budget allows you might also consider booking a private
VIP Tour for your day. This will allow you to maneuver through the crowds and enjoy the attractions of your choosing as you bypass the long standby lines. This is very pricey but it is something we did over the Thanksgiving holidays at Walt Disney World. There are also other interesting
Tours that you might enjoy versus just going to the parks for the attractions. This is another way to have a good experience during the busy times.
I hope you are able to develop a plan as it is so hard to be that close and not squeeze in a Disney visit!