We often travel over the 4th of July holiday, which is my birthday. (My twins' birthday is on the 5th.) The main purpose of traveling on these days is to celebrate our birthdays in the park. Believe it or not, this holiday isn't busy during the day. Traditionally, Independence Day is a day for barbecue and picnics. We stay away from the beach this day and travelers stay away from the park. Usually, we play all day and leave before the crowds begin to pile in around Disneyland to watch the firework display. (Yes, I'm a 4th of July baby, but fireworks are not my thing.)
You certainly will find the parks are busier during traditional school vacations times. If you can avoid going during winter break and spring break, you'll avoid most of the crowds. Christmas through the first week in January is a fun time to see the decorations, but be aware that you are going to be elbow to elbow with people the entire time.
If you're kids are younger than school age, try to plan your trip mid-week during the school year. The park might be more manageable than when there are significant crowds. Remember that Disney's California Adventure is much less crowded than Disneyland. Families with younger kids and strollers migrate to Disneyland. Believe it or not, thousands of strollers take up a lot of space.
If you're not in a hurry or planning your trip soon, download one of the wait apps for the parks. They will show you real time wait times for attractions and rides. This will give you an idea of what's going on at the park at that time. (We do this just for fun when we are not there too.)
The official Disneyland website also offers a wealth of information on when to visit the park to avoid the crowds. This also includes the park times which vary during the year as well.
You can find that information here: https://disneyland.disney.go.com/when-to-visit/
***Note, Disneyland and Disney's California Adventure are property of The Walt Disney Company. The opinions on these parks are simply mine. To find the most up to date and accurate information on the parks and their travel times, visit https://disneyland.disney.go.com