Delta will run a schedule change of some sort almost every weekend. Major schedule changes are much less frequent. Usually start sometime late Friday and will run through Saturday, depending upon how major the schedule load is. If it is a major schedule load, they sometimes have to lock up the seat maps until they have time to sort through who had what (although we all know that is not always a major concern for Delta). "Generally", once you get to within 60 days of flight time, the schedules do not change significantly, but that isn't a guarantee.