Crowd Calendar:
http://www.touringplans.com/tp2/UG2_index.php?PageID=14
from
http://mousesavers.com/
The best times to go to Walt Disney World are:
First three weeks of December.
The weather is usually cool but pleasant, the Christmas decorations are up, special events are running, and the parks are usually pretty uncrowded.
Early January through mid-February.
It's cool (sometimes even a bit cold) at this time of year. This is probably the least busy time of year, and hotel prices are lowest. However, usually some rides will be closed for refurbishment at this time of year.
Entire month of May.
The weather is warm in May, but usually not unbearably so. Crowds are low and hotel prices are moderate.
Late August and entire month of September (even Labor Day weekend).
It will be extremely hot. Plan on taking afternoons off and visiting water parks. Crowds and hotel prices are low. Off-site hotels usually have their rock-bottom deals during this period.
During special events that interest you. Bear in mind that events held on major holidays usually attract significant crowds, however.
The worst times to visit Walt Disney World are:
December 26-31. This is the most crowded and most expensive time of year.
Mid-February through mid-April (or later, depending on the date of Easter), especially holiday weekends including President's Day and Easter. Expect huge crowds, high resort prices and expensive airfares, but usually very nice weather.
Midsummer (June through early August). Expect it to be very busy and extremely hot. To maximize your experience, plan on hitting the parks first thing in the morning. After lunch, take an afternoon break, then return to the park in the late afternoon/early evening. 11:00 am to 4:00 pm is the hottest part of the day and ironically the most crowded, too.
Other holidays. Among the other busy time periods are Thanksgiving week and any national holiday with a 3-day weekend (except Labor Day weekend).
During special events. Some don't create a lot of crowds, but some do.
Some less-known events that may affect crowds, park hours and/or traffic at Walt Disney World: