One of my favorite times to go for Christmas is the first week in December. The park is not super busy because that is the time frame between the two major holiday school breaks. The weather during that time frame is perfect.
I went to Disneyland last year the week before Christmas, and even though it was crowded, we found ways to get around the crowd. The new MaxPass system was incredible. We were able to hit every ride we wanted too, plus our favorites multiple times! Honestly, I think the MaxPass is totally worth the $10 a day, especially when it is crowded like during Christmas time.
We went last year two days after Christmas and spent 3 days in the park, it was the first time we had traveled there other than in October. It was a bit more crowded, but was doable. The new MaxPass is so worth it, we were able to go on all of our favorite rides even though the lines were longer than normal. We just booked our next trip for December 31-January 4, 2019. Obviously the crowds after Christmas didn't bother us last year enough to still book during another busy week there. Hope this info helps!
I always tell people the great thing about going during a "busy" time is that there are very few closures. Disneyland tends to do refurbishments during 'off' seasons so that way more is available during busy times! Something to keep in mind