We go at this time every year (leaving Friday!!!!), and have never had "unbearable" crowds, except for at Magic Kingdom. There are lots of fun things going on, including the Flower & Garden Festival at Epcot and Star Wars Weekends at Disney/MGM studios. The holiday weekend itself is a bit crowded, and the following weekend is a bit crowded because of Gay Days, but it's lots of fun, and we just head to Epcot or Animal Kingdom when the other parks are too crowded.
As far as booking, I book right away at the 11 month window so I can have my Boardwalk View!