That answer depends on the seaonality of the grass. Personally I seed into the wet season because I am lazy and cheap, but even then I have learned the hard way, by being hard headed of course, that fescue planted in the first quarter dies in the third quarter due to insufficient root development before severe stress of heat and drought upon a cool season grass. Try to allow as much time as possible for root development. Toward that end, the sod used for the superbowl is started 18 months prior to the game, so 6 months of development before the quarter of stress season has to be some kind of minimum. Even then I think overseeding will be a future function.