I try to get my seed down before the heavy leaf fall starts beginning in late August or early September. My cut-off date is around November 15th but that is pushing it.
Why would you slit seed and also spread seed with a spreader? I either aerate and broadcast the seed or aerate and then slit seed. I will put shredded straw on bare areas but don't topdress. If you aerate and then broadcast seed, some will fall into the holes and some will end up under soil pulled from the holes. Aerating and seeding can help to thicken up a lawn but it's no substitute for slit seeding in my opinion, in most cases anyway. I have actually tilled bare lawn areas with an aerator before, but how well it works depends a lot on the soil type and moisture level.