This is not a simple question. Books and chapters of books have been written on the subject. The answer has many variables, such as do you want all the attractions open? Do you want lots of extended hours in the parks? Do you want to see parks decorated for special occasions such as EPCOT's Flower and Garden Festival (click here to read more), Halloween, or the holidays? Can you handle the heat? And, you confined to certain times of year to go due to work or school? See, the answer is complicated.
Disney devotes much time and energy in trying to predict the crowd levels and has designated certain times of year to various seasons--value, regular, peak, and holiday. If you are traveling during the summer for example, you could experience regular or peak prices and crowd levels. They both go up at the same time! During Peak and Holiday times, the crowds will be larger yet the hours of the parks will be longer and often there are more "mini' attractions like street shows or impromptu experiences that pop up to help with the crowds. More dining locations and times become available than during value or regular season.
Refurbishments typically take place during value or regular seasons. This is when park hours are scaled back, too. Of course, prices for hotel stays go down and often Disney special offers happen during these seasons. Value and regular seasons for the most part, mirror school calendars. When the kids are in school, they are not at Disney, hence value or regular season.
Here's when we have visited and our take on crowds, prices, temperatures, etc.:
November (around Thanksgiving): Crowds swell during this time, as do prices. The weather can be quite temperature and we did experience a cold snap. All parks are decorated for the Holiday season.
September (around Labor Day): Crowds are minimal, as are prices--often free dining is offered. The weather is still a bit hot a muggy. It's still hurricane season. Magic Kingdom is usually decorated for Mickey's Not So Scary Halloween Party.
March (towards the end of the month): If it is not Easter, the crowds are not so bad yet. The weather is great with cooler mornings and warm afternoons. Again, if it is not Easter, this is regular season so prices are reasonable and specials can be found. If it is Easter, all bets are off. Easter means Holiday season and crowds and prices swell.
November (in the middle): There is common knowledge that the entire state of New Jersey ascends on Walt Disney World the second week in November. The schools release in New Jersey for the state teacher convention. (We learned more about this on our last Disney Cruise as the family we dined with lived in New Jersey.) So, typically no specials are offered during "Jersey Week," but if you can arrive shortly after or at the tail end, specials can be had and the weather is wonderful. Crowds are pretty good, too. Typically the Magic Kingdom is decorated for Mickey's Very Merry Christmas Party by this time (Click here to read more) and Hollywood Studios has the Osborne Family Spectacle of Lights up and going (click here to read more).
Now when a location has 32 million visitors annually, know that there is never a time when it is not busy. Yet, a location that expects 32 million visitors annually plans to handle crowds and for the most part, Disney does that very well.
So, when is the best time? Anytime you want to go and can go. Just be prepared for the weather and crowd levels that you may encounter.
We are off again in November--regular season with free dining offer--for a 5 day quick trip. Our schools no longer have a break in March, so the next time we go will be in July. We've heard about the heat, but we'll get to experience it for ourselves.