There's a reason why Florida is called "The Sunshine State". On average there's up to 10 hours of sunshine each day, throughout the year it can get extremely hot and this should be a factor to consider when planning a visit to Disney World. (Also something to think about when choosing your suncream!)
The below figures give you a rough indicator on how warm it is throughout the year. Keep in mind though, it feels a lot different than the heat we have in England (whenever we see any). Remember this is just averages and it can vary a lot so we definitely recommend checking for the most recent weather reports for when you're looking to go.
It also does rain almost every day, especially in the second half of the year. At around 3pm you'll probably get a shower, maybe some thunderstorms, but they don't last long. And they're great to watch.
Peak Times & Prices
There are certain times a year when Disney World is really busy. Going at peak busy times does add to the cost of your holiday. Both Disney and airlines mark up their prices on school holiday's and other peak times. Disney also increase ticket prices (although you should be getting the unlimited tickets anyway) and also increases some restaurant prices.
General Times To Avoid If Possible
There are certain times of the year when it's much more busier than average, and should be avoided if it's possible. I understand it's not always possible, especially if you have kids. The only times you will be able to go is the peak times, like summer holiday's.
Times to avoid if you don't want it to be extremely busy include: Thanksgiving (end of November), around Christmas until after New Year, July, August (busy times with lots of tour groups), Labor day weekend in early September. Other times to avoid (unless specifically interested) is when Disney run special events like Disney Marathon in January and Epcot events throughout the year.
The last time to avoid, although this is much more rare, is as soon as new rides or park areas open. 2017 will see the opening of Pandora in Animal Kingdom, so expect crowds to be much higher in the park for the rest of 2017.
For a lot of people, the time to go is dictated by when you can afford to go. So keep an eye on the Disney World offers page. Off peak is the best time to go, but if you have kids, that may not be an option (but you'll probably be used to that if that's the case).
Off Peak Downsides
Although off peak times are the best times to go, prices are lower, it's quieter, there is one major downside, this is the time Disney close down rides for refurbishment. You can also always assume that the 2 water parks will be closed throughout the winter months, too.
Come On, Pick A Time!
I personally think the sweet spot is mid-September, and mid-May. You get the lovely heat, but it's not too overbearingly hot like it is July and August. Of course, depending on your party, budget, and heat tolerance, and everything described above, only you can decide what time of year is most accessible to you. Whenever you go, you'll have a great time!