Unfortunately, it looks like all of March is considered Peak Season, must be all those spring breaks.
These are the dates that Mousesavers has listed for 2004:
Value Resorts
All-Star Movies
All-Star Music
All-Star Sports
Pop Century
Value Season: January 1 - February 11; August 29 - September 29; October 31- December 19
Regular Season: April 18 - August 28; September 30 - October 30
Peak Season: February 12 - April 17
Holiday Season: December 20 - 31
The moderate resorts have the same seasons but the deluxe resorts have a slightly different schedule.
Hope that helps.