We go at least twice a year. The worst time we have ever went was in March at Spring Break, never again. All the Florida kids are out by June and that is when we usually go. I think your question may depend on what park you are thinking of. To me Magic Kingdom is always busy in the day and it does not seem to matter what day of the week it is?
I think you'll find that the weekends are busier than weekdays, no matter what time of year it is. You not only get "locals" on the weekends, but lots more people plan long weekends to take advantage of their days off.