We're going to California March 4-16 because it seems to be very quiet this year. Since Easter is in April, the busiest crowds are expected the first two weeks of April.
The March calendar for Disneyland is up on-line, and you can gage the crowds by their hours of operation. The week of March 4th is extremely quiet, with hours starting to be extended on the weekend of March 10th.
According to the Legoland, the month of March is expected to be quiet. And, San Diego Zoo posts it's official spring break dates as March 31 - April 15th.