While Disney won't discuss actual attenance figures they do release the following for planning purposes.

The Lowest Attendance:

• January (except New Year's Day) until just prior to Presidents' Week in February
  
• The week following Labor Day until the week prior to Thanksgiving
  
• The week following ThanksgivingÊuntil mid-December
  

More Moderate Attendance:

• After Presidents' week in February through early March
  
• Late April through early June (except Memorial Day weekend)
  
• Columbus Day Weekend (October)
  
• The first part of Thanksgiving week
  

Highest Attendance:

• Presidents' week in February
  
• Mid March through Late April ("Spring Break")
  
• Memorial Day weekend
  
• Mid June through Labor Day
  
• Thanksgiving Day and weekend
  
• Christmas week through New Year's Day

Another great resource to decide when is the best time for you to go to Disney World is the Week Picker App (2010 version / 2011 version) at yourfirstvisit.net