I've been asked several times recently about fall seeding. I've even done a little myself. The main concern is in the timing. Is it too late, is it too early, and so forth.
True, seeding is generally recommended in the fall but it's a long season and there are many different types of seed.
It's best to seed early in the fall. It's important to allow the seed plenty of time to germinate and establish itself before cold temperatures and frosty mornings become the norm. Young seedlings are sensitive to extremes and could be damaged or killed during periods of frost or during winter.
Make sure you know the germination times of your seed. A quick starter like Perennial ryegrass can be germinated in 5 days and mowed/established in 21 days. A slow-poke like Kentucky bluegrass takes much longer to sprout and may not make it through the winter if planted too late.
There's also what's called "dormant seeding" where the seed is sown very late in the fall and left in place to germinate in the spring. This method is perfectly acceptable but the risk of something happening to the seed is quite high. Birds eating the seed, rain and melting snow washing the seed away, are a few of the factors that could reduce the success of a "dormant seeding".