There are 3 times of the year that are "slow". When I say slow, I mean that I can walk onto rides and the longest wait is 20-30 mins.
The last week of April, the last week of August/first week of October, and the week before Thanksgiving. We do try to avoid going on weekends though so that makes a difference in crowd levels.
we went the last week of April one year when it coincided with the Patriots holiday that a lot of New England schools get off and it was BUSY! Touring plans rated one of the days a *10* after the fact. So that week may be low crowds sometimes, but you need to be careful with it.