Well I know that's not Golden Week (a week of several Japanese holidays), which is at the end of April/beginning of May and I know the Tokyo parks are PACKED that week and to avoid at all costs.
But is is the typical "spring break" week for Japanese schools, which is from March 22 to April 7 in 2014. SO you can count on the parks being VERY busy. Plan on getting to the gates at least an hour before park opening, bring your patience and play the Fastpass game to your best advantage.