These are the main holidays discussed above. Most of them are not always a set day of the year (like Christmas is December 25th), but are a set day of the week.
Martin Luther King Day - The third Monday in January
Presidents Day - The third Monday of February
Easter - Third Sunday of April
Mothers Day - Second Sunday of May
Memorial Day - Last Monday of May
Fathers Day - Third Sunday of June
Independence Day - 4th of July
Labor Day - First Monday of September
Columbus Day - Second Monday of October
Halloween - October 31st
Veterans Day - November 11th
Thanksgiving - Last Thursday of November
Christmas Day - December 31st
New Years Day - January 1st
So..... generally speaking most US Holidays are observed on Mondays - making for 3 day weekends (Saturday, Sunday, and then the Monday). These weekends are almost always much more crowded than the average weekend. MOST US School years run from around Labor Day to around Memorial Day. During this period, you will generally find lower attendence. The exceptions during this period is the week of Thanksgiving (From The Saturday before - to the Sunday after), when most schools are out. Also, the week of Christmas - until AFTER New Years Day is also crowded because again schools are out.
Of course there will always be some crowds at WDW. In 1994 I was there during the week of Thanksgiving - and it was a madhouse. Last year, we were there the second week of September, and many rides were walk-on. When planning - and looking for a slow time - just try to pick times when kids will be in school. It is during these times that WDW will be less crowded.
Hope this helps - and if I messed up on my Holiday Conversion Chart, I'm sorry...... I don't have a calendar in front of me.