Just weighing in on this, but I could also see a value of having a program run between specific dates.
As William alluded to in the thread here, the city of Orange, CA has as part of it’s watering restrictions, limits on the number of days per week one can water that changes from one day per week in Nov-March to two days per week in Apr-Oct. This could easily be achieved with two sets of programs for each of the corresponding range of months. There are other cities in California that also have similar day per week changes during the year that could benefit.
Another use case that I personally would benefit from is related to having a different watering schedule that would be used for two weeks immediately after planting the vegetable garden (to allow the plants to take root and establish), and a regular schedule that would be used for the rest of the growing season. As with the previous example, this can be accomplished by manually enabling and disabling two different programs, but it would be great to have this automated by schedule.