Crowds are lowest during September, early November, early December, May, and in January and February (excluding the government holidays of Civil Rights and President's Day). But keep in mind, Disneyworld and Universal Studios are never empty. The very quietest months at the theme parks tend to be September and May, as families with school-aged children settle back into their school time routines in September, or postpone travel until school is out after May. January and February have some weeks that have relatively low crowds, but watch for scheduled events like the Disney-sponsored marathons or team sports competitions, when Disney successfully brings a few more guests to the parks in an otherwise relatively quiet couple of months.
Because of fall breaks from school across the nation and the gorgeous Florida weather, October is a popular time to attend theme parks, but the crowds are not prohibitive for the typical tourist.
The most crowded times at the theme parks are Thanksgiving week, Christmas & New Year's weeks, Spring Break/Easter Break (mid-March to mid-April), and summer from the second week of June, especially in the whole month of July, and the first week or two of August. But don't lose heart is this is the only time you can travel because of school schedules; it's the obviously the same for most guests! Keep reading for tips to enjoying the parks if you are travelling to Orlando during on of these peak seasons.
For the 4th of July, Magic Kingdom will often max its capacity, as guests love to see the holiday fireworks. Any weekend days or evenings will ALWAYS more crowded, regardless of the time of year, but exponentially so during a school holiday week, too.
Disney's Magic Kingdom and Hollywood Studios, as well as Both Harry Potter sections of Universal Studios seem to be the most challenging parks when crowds are heavy. Sea World is always great...even during holidays, crowds are not prohibitive.