Mid August to early September is the best time of the year to seed grass. Whether you are over seeding thin areas, seeding smaller bare spots, or seeding a large area, right now is a great time.
One key to success in preparation. Eliminate weeds in the area, rough up the top inch or more of soil with a claw, hoe or stiff tined garden rake. Grass seed will germinate much better and faster when it is gently worked into the top 1/2″ of crumbly soil. Grass seed lying on the top of bare soil germinates poorly, and the same is true if grass seed ends up deeper than 1/2″ deep. Excellent germination is usually the result when the seed is mixed into the top 1/8″ to 1/2″ of soil.
Another key is to keep the seeded are moist. I recommend a light watering early to mid-morning, and a second light watering mid to late afternoon. If you can keep the seeded area consistently moist for 10 days, you will most likely have a wonderful stand of grass.
On slopes the addition of an erosion control blanket may be wise to prevent erosion resulting in gullies that require more soil mix and seed.