Hi Joanne!
I like to start off by looking at the park calendars and an estimate of the crowds for the different theme parks on different days. Then I adjust my schedule to include any special events, Extra Magic Hours that I actually plan to attend, etc.
You can find out when any parades, fireworks, and major shows will be offered in one of the Walt Disney World theme parks by looking at the park calendars for the Magic Kingdom, Epcot, Hollywood Studios, and Animal Kingdom. You can also find out when the park will open, when it will close, and if it will be offering Extra Magic Hours that day. The information can be found by navigating the menus on the main Walt Disney World
website. Click on "Parks" at the top of that page and then go to one of the theme parks. Once you are on a specific theme park page, you can click on "Calendar" on the left and pull up a monthly calendar for that specific park. Go to the month you are interested in, and then click on the days you are interested in to see the park hours, etc. Disney tries to have the information loaded into the calendar about six months in advance.
I then visit the independent website TouringPlans.com. That's the website for the research team behind the tour book "The Unofficial Guide to Walt Disney World". They are famous in the fan community for producing a crowd calendar with an estimate of the crowds in the different theme parks on a particular day. Their predictions are based on years of collected data and a lot of math. As an engineer, I'm a huge fan of their work. They are also famous for producing touring plans (basically a sequence of experiencing the attractions and shows) that minimize your wait in lines. Definitely check out their website!
I hope this helps!