There are a few principals that will help you seed at the right time. First of all you want your newly seeded grass to be moist, something that's practically unachievable during summer. Second of all the heat helps the weeds grow and therefore once more you can see that summer isn't the greatest time to plant grass.
You should start planting at the end of summer. That way the temperatures won't be that high and it will be easier to keep the grass moist. Your main goal is that by winter time, the grass will be strong enough to survive the cold.
Small adjustments to the time of planting should be made in accordance with where you live and how fast winter sets in.
May your grass always be green.
