Public schools throught the country usually have either the week before Easter or the week after Easter off. This is when people with children in school feel it is "safe" to take the kids on a week vacation and not affect their schoolwork.
During these two weeks Disney goes to additional hours and there may be Stage Closings in the Parks due to crowding. They are usually able to fill just about all Resort rooms without discounts.
Is there any valid business reason for Disney to offer discounts during this period?
And, yes, they are offering discounts starting the Thursday after Easter, but this is for new reservations and by then they are starting to pick up families that are not looking for Spring Break days off from schools.