Hi Carol!
Thanks for your question. Seasonal dates have always been sort of a moving target at Walt Disney World, but they seem to have leveled out in the last year or so with regard to consistency. For 2010, the seasonal dates are:
Value Season (or least expensive) runs from January 3-6, January 10-14, January 18-February 11, July 18-September 30, and November 28-December 16.
Regular Season is April 11-July 1, July 5-17, October 1-7, and October 11-November 13.
Peak Season is February 15-March 27 which includes Spring Breaks throughout the country, and
Holiday Season is January 1-2, January 7-9 (WDW Marathon), January 15-17 (Dr. Martin Luther King, Jr. holiday), February 12-14 (President's Day), March 28-April 10 (Easter), July 2-4, October 8-10 (Columbus Day Weekend), November 24-27 (Thanksgiving) and December 17-January 1(Christmas/New Year's).
In addition, the resorts have begun charging more for weekend night stays (an additional $25 in most cases) regardless of what season the nights fall under. I hope this information helps you in planning a magical vacation ! :)