Whether it's over-seeding bare patches, renovating an existing lawn, or starting from scratch, grass seed will establish better if it's planted in the fall. This is true in spite of the fact that most people believe that spring is the time to seed a lawn. There are assorted reasons why fall is better. Most lawn seed mixtures are high in bluegrass seed. Under the best possible conditions, bluegrass takes three weeks to germinate.
Bluegrass grows best in cool weather.
Planted in the spring, grass seedlings immediately plunge into hot, humid summer weather, which is precisely what they hate the worst. Planted in the fall, grass has a long stretch of cool weather before the winter enforces top dormancy, then there is another cool growing season in the spring before summer clamps down. Equally as important, the key to a fine lawn is developing a sturdy, dense root system on the grass. Plant roots continue to grow throughout the winter whenever the soil is not actually frozen. The fall planted lawn has maybe eight months to develop strong roots before it has to face the feared and hated summer.