Okay, OP You said December was out....so let me see if I can help....Let me start by saying that crowds should not be your only consideration: aver. daily temperature, hotel rates, and activities should also be a factor.
Late summer is slower time of year, as far as I know, but then it is very hot as another poster said. Not so much fun with little ones. Also late summer/fall is hurricane season, also not a fun thing to get caught up in.
School (K-12) vacations are going to be the worst times to go, between them will be better, even during more crowded times of year (I'm esp. thinking spring break season).
February is not a bad time to go. Late January the crowds will be lower, but the weather will be really cold, and you may miss out on numerous things - like the awesome pools and the water rides. The resort rates go up after Valentine's day. So if you check in on the 13th, you'll get the lowest rates, on the other hand, it can still be a little cool that week. President's week will again be crowded, but not overly bad. Those months, you also run the risk of snow - not so much in FL, but in MA. If a big Nor'easter hits, (assuming you will be flying) you might get stuck in Orlando a few extra days, paying top dollar for a room. Also, Daytona happens that time of year. Usually there isn't too much overlap, but a little. Late March the weather is warm, it's great to see sun after a cold winter, but some crowds will be there.
The tail end of Peak season is April 14, so April 15 things will pick up a little, but probably the last week of April/start of May won't be bad this year. (Though mid-May isn't too bad). April 5th is when the flower festival begins, and it's very long this year, so even though you'll pay top dollar for your room, The week after Easter (April 8) probably won't be too bad, and you'll get the flower show (Avoid the week before Easter, in particular). At some point there are the Star Wars weekends, but I don't think the crowds will be bad anywhere but MGM on those weekends (during the week it would probably be okay) WE went once during super Soap Weekend. The park was pretty crowded, but the ride lines were actually quite short. We pretty much walked onto Star Tours, even though the crowds outside were a little hectic.
Disney has been doing lots of research into slow times of year, and is always trying to come up with ways to increase their attendance durig those times. They keep getting better at it...whatever times you are thinking about, check with Disney and see if there is some special event going on while you are there. Usually, if it's a slower itme of year, it will just mean modifying what day you go to each park.
