From 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.
Other holidays. Among the other busy time periods are Thanksgiving week and any national holiday with a 3-day weekend (except Labor Day weekend).
Some less-known events that may affect crowds, park hours and/or traffic at Walt Disney World:
Cheerleeding/Twirl/Dance Competitions - Various dates each year, usually January-March. These usually don't have a lot of impact on the in-park crowds. The issue is that the Value resorts (All-Stars and Pop Century) play host to hordes of girls who can be extremely loud -- so you may want to avoid those resorts during competitions. Some of the competitions include College Cheerleading and Dance Team Championship (January 1316, 2006), National Dance Team Championship (February 36, 2006), National High School Cheerleading Championship (February 1013, 2006), Twirlmania (February 1720, 2006), Cheer & Dance National Championships (February 2526 and March 35, 2006) and National All Star Cheerleading Championship (March 1013, 2006).
Mardi Gras Week. February or March (varies annually: February 28, 2006; February 20, 2007; February 5, 2008). Many New Orleans schools have all or part of this week off, so families leave town the weekend before the holiday and head to Disney World in droves.
Grad Nites - Usually the last two weekends of April. (2006 dates: April 28-29 and May 5-6.) Private parties for graduating high school seniors are held from 10:00 pm through 4:00 am at the Magic Kingdom. The park closes to the public at about 6:00 pm. From 8:00 pm on, avoid the area around the Magic Kingdom as hordes of buses will tie up traffic for hours. Also, be aware that some resort guest rooms facing the Magic Kingdom may be affected by amplified music coming from the park until 4:00 am. If you're staying at one of the Magic Kingdom resorts on a Grad Night, you may want to ask for a room on the opposite side from the park.
Gay Days. First week of June. Visit GayDays.com and OneMightyWeekend.com for more info about the two organized events. At most, only one park per day will be more crowded than normal; often there is no discernable affect on crowds. For 2006, park attendance is most likely to be affected on June 3 at Magic Kingdom, because that is a busy summer Saturday and both groups plan to visit on that day.
"Jersey Week" - Usually the first or second week of November. The New Jersey schools have a teachers' convention (upcoming convention dates: November 9-11, 2006; November 8-10, 2007; November 6-8, 2008). Lots of people pull their kids out of school for the whole week and add on the weekend before and/or after, taking this opportunity to go to Disney World in the "off season." New Jersey is the single biggest market for Walt Disney World -- you do the math!
Pop Warner Week - The Pop Warner Superbowl and National Cheer & Dance Championships are held at Disney World in early December. (Next event: December 3-9, 2006.) This may not affect crowds in the parks much, since the kids will be busy playing in the championship games, but the Value resorts and some of the Moderate resorts will be FULL of kids (both football players and cheerleaders). Expect a lot of noise if you stay there