Currently no. You would have to change your intervals to day of week, versus a true interval (1-21).
Using day of week might be better suited right now, something like Monday/Wed/Sat?
But...our new software 2.0 release will have the concept of flex schedules. You tell us what days you can water on, and we take care of the rest, only watering when we determine your yard needs it, and not on Fridays
Targeting end of April for 2.0 mobile/web release.
Let us know if you have any more questions or feedback.
Have a great day!