Fall is the best time to grow grass seed in the North, but you can also do it in early to mid-spring. According to the University of Missouri Extension, grass should ideally be planted in September so that it has time to become established and thriving by the following spring.
Spend August preparing the yard for new grass seed. Use an herbicide to remove weeds and loosen the soil to a depth of 4 to 6 inches. Have the soil checked for fertility deficiencies.
If you must plant grass in the spring, make sure it's in the earlier part, once the danger of frost has passed. Sowing in late spring can be problematic because the lawn won't have an opportunity to grow strong roots and mature before the heat of summer arrives.