DO know that August 15 to September 15 is the very best time to do lawn repair in northern Illinois, whether seeding totally bare areas or overseeding thin lawns. Yep, actually better than spring due to less weed competition.
DON'T use a broadleaf weed (dandelion, clover, etc.) control to kill weeds before seeding. There's a lengthy restricted time between using these products and being able to safely reseed.