The first two weeks of September is the best time to reseed your cool season lawn.  Do not wait! Use 6 pounds of seed/ 1000 square feet.  Choose seed with a germination rate above 90% and with as high a percentage of guaranteed seed 97% or better as you can find.  Plan to water 2-3 times a day at first for the first 2 to 3 weeks then gradually reduce the number of times a week you are watering to once a week (by the third mowing). The grass needs about an inch of water and the water needs to be reaching depths of 6 to 8 inches deep.  This will promote deep rooting and protect the grass from long periods of dryness.