Plant grass seed in Denver anytime between March and the first expected fall frost, according to Colorado State University. It depends on the type of grass. When planting Kentucky bluegrass, disperse it between mid-August and mid-September. The second best time is March or April.
When opting for warm-season grasses, such as buffalograss, sow between April and July. Avoid dispersing grass seed after July. Do not use a warm-season grass if you live at an elevation above 7,000 feet,
Time is needed in advance of planting to prepare the soil. According to Colorado State University, poor soil preparation is the main factor in lawns failing to germinate. Till the soil and add compost or manure to make the soil more fertile.