Welcome to the forums danielle h,
Its extremely busy at Disneyland the full week before Easter Sunday, as most Secondary and Elementary Schools in the U.S. take off that week for a Spring Break. With some taking the full week After Easter Sunday.
Most of March and April are full of College Spring Breaks, as well as its a popular time for Conventions and School Competitions at the Convention center which is right across the street from Disneyland, and alot of those will also visit Disneyland
If you need to go in March, usually the first week is the only time that would still have the lower crowds.
If you need to go in April, the last week of April would also usually have the lower crowds.