As long as the soil temperature is 50 degrees for higher, you can plant seed. However, the best two times are in the spring, when the soil is moist, and in the early fall, when temperatures are still warm enough for the seed to germinate, and there is still enough time for the grass to get established before the first frost. Fall has an advantage because the new grass seedlings won’t be competing with common lawn weeds which are starting to go dormant at this time.