Cool weather is the best time to plant cool season grasses such as Kentucky Bluegrass. The optimum dates in Denver are between mid-August through mid-September. This allows the grass to establish itself through two growing seasons before the heat of summer.
March through April is the second best time for planting cool-season grass.
April through July is best for warm-weather grasses such as buffalo grass. Do not seed after July or if the elevation is above 7,000 feet.