I don't know for sure - just guessing, but I would imagine that the work is year round if you take into consideration that the writer's are probably working before and during the normal hiatus time for the actors. Possibly at the same time, a lot of the pre-production people are also working during that time finding location shots, doing costume designs, set designs, and such so that everything is ready by the time the actors have to stand in front of the cameras again.
Cheers,