According to Purdue University, between August 15 and September 15 is optimum grass seeding time in the northern half of Indiana.
A combination of warm soil, rainfall and lack of weed production make it ideal. The moderate fall enables newly planted grass to get established with two growing seasons before the possible harsh and hot summer season dries it out.
Seeding at the beginning of the season gives the grass even more time to become strong. Waiting even a week can make a significant difference.