Hi Madelon!
In general, the value season at Disney occurs when school is in session and crowds are at the lowest. This adds up for great prices and lower crowds: What a great time to visit!
The value season for the different resorts does vary slightly based on the type of resort (Deluxe, Moderate, Value). The best way to check on dates is to visit a resort page on the DisneyWorld.com site. Once there, click on the link for: Room Types & Pricing. The drop down box in the upper right will allow you to choose the 2011 Value Season dates.
For example, the Animal Kindgom Lodge
Room Types & Pricing page has a listing for the 2011 Value Season - Weekday Rates as follows:
August 21, 2011-August 25, 2011
August 28, 2011-September 1, 2011
September 4, 2011-September 8, 2011
September 11, 2011-September 15, 2011
September 18, 2011-September 22, 2011
September 25, 2011-September 29, 2011
January 17, 2011-January 20, 2011
January 23, 2011-January 27, 2011
January 30, 2011-February 3, 2011
February 6, 2011-February 10, 2011
February 13, 2011-February 16, 2011
January 9, 2011-January 13, 2011
January 2, 2011-January 5, 2011
Be sure to check on the resorts you are interested in staying at and compare the prices.
Hope this helps!