Spring Breaks vary by school districts, so there really isn't a set "spring break". I think the majority of school districts probably have their Spring Breaks sometime between early-to-mid March through late-April in 2017. Based on the SoCal Select AP Blockout calendar, I'm guessing Disneyland expects to see the majority of the "Spring Break" season crowds from March 17 through April 23. Note that Easter is late in 2017 (April 16), but not all schools base their Spring Break on Easter.