I was taught (turf management course) similar to Evan's Penn State link, late summer/early fall is the best time for seeding. The reasons given were that the soil is warm and there is less weed competition than in spring, and the seedlings should be up and running and hardened-off in time for winter. Also by the time the hot dry weather comes around again in the following year the plants have already rooted deeply, whereas spring sown ones are still shallow-rooted and the stand is thin so the soil temperature gets higher due to lack of shading from each other.
It was suggested that the third best time after late summer/fall, and then April-June (here in Ontario, probably earlier in PA) was what is called "dormant seeding" (mid-November here). At that time the seed doesn't germinate but waits until spring. This should only be on level ground with a straw mulch otherwise the seeds can get washed away when the snow melts. I guess this is similar to what Woofie suggested.
I have early-fall seeded and it worked very well.
