depending on the grass some grasses need a freeze thaw cycle, on the seed before it will germinate,
fall is many times the best, and in some grasses it may take up to 6 years of natural process to germinate,
on many of the government projects such as CRP, in our area they say do not even be concerned over the grass stand unless three years have passed, and I have noticed that some areas have needed to go ten years before the grass is of a reasonable stand and some of it took 8 to 10 years to come up,