The plans recommend replacing the Sunday SS session with a longer endurance ride if preferred. If you dive into the ‘Week Tips’ they suggest alternatives for Sunday sessions with endurance workouts, so no reason they can’t be done outside.
Looking for something a little less intense and don’t mind some extra time in the saddle come Sunday? Have a look at Town Hill or something similar.
Either day would be fine though - if you want to ride outside on Saturday, go for it - just complete Saturday’s interval session on the Sunday.