Basically, there is no perfect crowd calendar, since there is really no telling when a place will be crowded or not. There is just too much going on all over the world, that there is no telling how it will be on any given day.
Most will agree that month to month, roughtly expect it to be like this:
January- after new years, its one of the slowest times of the year.
February, mostly decent crowd levels, but avoid Presidents Day week when it can fill up big time
March- mostly decent, but can fluctuate with various "Spring Breaks". Since spring break is different everywhere, expect crowds at times throughout the month. Stay away from easter.
April/May- mostly good, avoid easter/memorial day, but nice times to visit outside of these, lighter crowd levels compared to summer, and nice warm weather.
June/July/August- the kids are out of school, the parks will be crowded. No ifs/ands/or buts, excepting maybe right at the end of August when most kids go back
September- right there with January as one of the slowest times. Kids just went back to school, most parents don't want to pull them out during this time. But, I'd avoid Labor Day week.
October/November- In general, "lighter, manageable crowds compared to summer, BUT realize Halloween/epcot food and wine festival will attract some crowds in this time period.
December- very light crowds until middle of month the week before Xmas- then its absolutely insanely full until New Years.
All of this generalization aside, one golden rule- be aware of all holidays. If your kids have a day off because of "holiday X/Y/Z", and you are thinking of visiting because it would be convenient because of them missing less school time- count on a million other people having the same idea. Even outside of the obvious big ones "Xmas, Easter, Thanksgiving, etc. The park is least crowded when you probably least desire to go (January's post holiday "dead of winter" times, and September (right when kids go back to school, but for the love of god, NOT labor day week). If you can manage it and want to experience as light crowds as possible, go during these times.