Overseed now. You need ~45 days for the roots to establish before frost/freeze. That puts you into mid-Oct now, which is probably after your 1st frost. Dethatching is a must for overseeding. You want to clear as much debris as you can to expose the soil below. Without good soil contact/cover, you risk the seed washing/blowing away and taking longer to germinate. The dethatchers you rent are scarifiers, that will also open up the soil for the seed to drop into. I also recommend running the dethatcher after seeding to cover the seed. Make sure to use a new seed fertilizer. And lastly, water, water, water the new seed. 2-3 times a day lightly at first to keep soil/seed moist. After it's germinated you can go longer/deeper and further between with watering.