Summer is when families with children head to Disneyland in droves. The weather is perfect (there's a reason so many people live in Southern California), the days are long and the kids are out of school. It's an exciting time to be at the park, with a never-ending parade of activities. For holiday events, the best time to go to Disneyland in December is the week before Christmas through New Year's Day. This time of year is very busy, as is spring break (around Easter). Sure, you're going to wait in some long lines during these peak periods (take something to entertain the little ones during the wait), but that doesn't seem to keep anyone from having a good time.
November to early March is the coolest time of the year, and in general, the best time to go to Disneyland to avoid crowds. Temperatures range from lows in the 40s to a high of 70. Lines are the shortest from the second week in January to Presidents' week, as well as two weeks after Easter Sunday through Memorial Day week. One other option for those who want to avoid long lines is to visit anytime from the end of Thanksgiving weekend through mid-December.
If you can't stand crowds, the best time to go to Disneyland is in October and September. The weather is spectacular (high in the low 80s), and the crowds are manageable, since most children are back in school. The time between Memorial Day and the beginning of summer vacation also offers moderate temperatures (mid-70s) and lighter crowds.