Not exactly sure why, but when we have been in WDW for Easter the parks are usually less crowded on Saturday and Sunday than the days before and after. My reasoning for this is
1. People take their vacation from Saturday to Saturday and therefore are not in the parks on Saturday (or for maybe half a day). They either want to be back home for Easter Sunday with their family OR
2. Many people go to church on Easter Sunday and do not plan parks if they are here.
This is just my thoughts, of course, but in our experience Saturday and Sunday over Easter have the lowest crowds of the 2 week period. We will at MK this Easter Sunday.