Now that I've booked my flights, I can honestly say that we'll be there Apr.16-23, which is spring vacation for many New England and some New York schools. We went the same week in 2004 and thought it was moderately crowded, but it was also right after Easter, so who knows? Easter is earlier in 2005. We did Early Entry and FastPass and park hopped a lot, hit MK every evening after early dinner, so we pretty much got to see and ride what we wanted.
Grad nights for the high schools were that week in 2004. The MK closed around 6pm on ( I think ) Thurs. and Fri. so that the local high schools could have their grad night parties. So be sure and get your MK touring in early in the week if grad nights are that week again. I haven't checked out the dates yet.