I agree with Bob - if you go in February (but NOT during school break week or around President's day weekend) the crowds will be about the same as after Labor Day in September. The biggest differenct is the weather. We've been in Jan/Feb several times, and we've had 45 degree days and 85 degree days (in the same trip!). September varies from kinda hot, to very hot, to mop me up off the floor I'm melting hot. I like going in September, because we get one last shot at summer activities before the winter sets in, but there's something about leaving the cold NE heading to "warm" 50 degree weather that just makes me smile!
So, if you don't mind the heat, crowd-wise September is a great time to go!