Some areas of Canada and the N.E. U.S. begin in February. From mid Feb through early April it's SB somewhere. If trying to avoid the crowds, kids and 20 somethings, you'll want Jan or late Apr/May. In June through Early Aug., summer breaks happen.
If you're looking to join the party with the above crowds you have a large window.
P.S. if anyone out there is playing the waiting game hoping rates drop last minute on the Spring Break or summer vacation time frame, I'd advise against it. Most sailings on the better ships climb the closer you get along with airfare pricing. Cabin availability goes away fast for desirable spots. And if you'll need any cabins for more than double occupancy, connecting or side by side cabins you may already be too late for many dates. The ones still available are climbing in price as the availability goes away for these family friendly cabins.