If you have been to Disneyland in the last few months then you know how crazy busy it can get there. You could wait 2 hours to ride Space mountain! That being said the LA Times ran a study on the best times to go to Disneyland!
They said the best day to go is on a Thursday. While The best months to plan a trip are on September and January.
This link has more on the study. http://www.latimes.com/business/la-fi-disneyland-wait-times-20170712-htmlstory.html