Holiday season is the best time at Disneyland, however, this time of year also brings in some of the busiest crowds! Duh!
Thanks to the Disney Tourist Blog, they've constructed a crowd calendar based on the best and worst days to visit the theme park in December. Through analyzing Christmas vacations, school breaks in the LA and OC counties, & tourist numbers, they've predicted the traffic of the park.
Based on the calendar, if you want to avoid crowds, going Monday through Thursday are your best bet to beat the peak of the crowds.