Actually, there are two good times. In late winter or early spring, as soon as weather permits, is one. Seeding of thin established lawns can even be done on frozen ground, where winter’s freezing and thawing create a “honeycomb” effect in the soil, providing a good place for the seeds to lodge. For seeding new lawns, however, soil must be tilled and dry first.
The other good time to seed is in late summer or early fall—around Labor Day. Seed will germinate when daytime temperatures are consistently 68° to 85° F (soil temperature approximately 55°). Summer seedings, although not recommended, can be successful, provided you are able to keep the seedbed constantly moist by repeated waterings