If you want the most accurate crowd predictions, take any WDW Crowd Calendar and throw it in the garbage, as those can be very misleading. Instead, look at the schools/colleges and their calendars. Find out when the majority of schools/colleges/major universities will be on vacation, and when they'll be in session. THAT is how you plan.
For instance: Do you want to visit during the best times of year to avoid long lines? Then look at when schools will be engulfed with final exams; or just before letting out for a major break/holiday. Times such as these include: The week prior to Thanksgiving week, the very 1st week in March before Spring Breaks start to take form, early/mid-May before Memorial Day weekend. etc.
Also look at when schools just go back in session from a major break/holiday. Times such as these include: Mid-late August when most schools/colleges are just beginning after summer break. These are all great times to visit the parks if heavy crowds is not your cup of tea.
When planning a trip, ask yourself: What will the schools be doing when I'm going? Will they be in session, or will they be on vacation/3-day weekend? If you're going during the week of Christmas - New Year's, the answer to this should be quite simple, as every school & university in the nation will be out, and people will be traveling! Expect crowds & lines during that time. Spring Break is a little tricky since school systems across the country differ on which week they will be out. Any time from 2nd week of March thru end April could be hit or miss with these crowds.
In simpler terms, avoid weeks/weekends which have a title to them (President's Week, Christmas Week, MLK Weekend, Thanksgiving Weekend, etc) if you are not a fan of the heaviest crowds.
Or this little rule of thumb which is easy to remember:
-When the schools are out, you're gonna pout.
-When the schools are in, the lines are thin!
