Fall is a time when more root production is going on. Aeration in the fall helps establish the lawns root system. Come the next spring and summer they will see the difference. I agree that spring is a good time too, but most of mine are done in the fall. The comment of too wet in spring is really true for most northern lawns. If you wait too long in the spring, the weather gets too warm. Sure you can aerate anytime the grass is growing, but the stress and heat for the average, non-irrigated lawn is too much. I like fall.