Overseeding refers to sowing the seeds of a different species into your lawn to improve its durability and appearance. For cool-season grasses, the best time to overseed is fall.
Fescue or Kentucky bluegrass responds well to overseeding in the fall because the soil is still warm but the peak heat of summer that cool-season grasses tolerate less is dropping.
The seed will have enough time to establish its roots before going dormant after the first frost. There will also be less competition from weeds this late in the season.