Farmers generally do their frost seeding of clover when the winter is almost over, but there is still some freezing going on, in our zone 6b that's late February. Seeding too early in the winter can lead to lower germination rates. When overseeding, the key to frost seeding is how much soil is exposed in the existing vegetation, if there is a heavy thatch it should be scarified or scratched with a chisel type implement to allow for better seed to soil contact. That's why a lot of overseeding is done with an interseeder or notill drill, which cuts through the thatch and dribbles the seed on the exposed dirt.