WDW does not release attendance records.
Below is a general list of most crowded to least list.
1. Week between xmas & NYE
2. Thanksgiving & Week of Easter.
3. Summer, starts Memorial weekend and goes about a week after labor day.
Least Crowded Time periods: January after the 5th, Sept, Oct and the first few weeks of Nov & Dec.
You also might check the following site:
http://www.allearsnet.com/pl/attend.htm