honestly it depends on the weather...you could seed as late as November if it stays warm enough but it is usually better to get your seeding in before all the leaves start to come down and you really begin your fall clean-ups...i'd suggest no later than the second week into Oct.
I'm starting my aerating and overseeding jobs next week and will have them all done within 2 weeks. Don't want to push it too much since the weather this year has been so unpredictable. The closer you get to the first frost or bad cold snap the more of a chance you're taking.
Here's an odd little note. I grew up in Wyoming and the growing season was about half of what it is here in Kentucky. My dad would start reseeding his lawn as soon as he saw the first snowflake. He didn't care if it turned into a blizzard; you would see him out there with the spreader. Always had the nicest looking yard in the town.
all kentucky blue and fesque should be seeded by oct. 14 i believe and perr. rye by the 30. i have seeded in late nov. 2 years ago and had great germination it was suppose to be a dormacy seeding but it was in the lower 70s and mid 60s for 2 weeks. then in the mid 50s for the 1st week of dec. even odder the customer called me dec. 12 and asked me to come cut the yard. it was high.
The so called "best time for seeding lawns in central Ohio is @ Aug 15-Sep 15. Most seed should be in the ground by Sep 20. The season is a little extended in southern Ohio and shortened in the north. Of course these are just guidelines and alot is dependent on how the weather holds out that particular year. Also, the germination times required for different grass types are a factor. The problem is the potential for new seeddlings to be hit with a hard frost are early freeze. This is a big problem for tall fescue but luckily it has a quick germination. The more mows you can get on the new grass before winter, the hardier it will be.