Berts, this issue is addressed in our document Getting Started with 80/20 Training but I have include the relevant section below for your convenience:
Q: How can I change which day of the week my rest day is?
A: 80/20 plans don’t assign a particular day of the week as a rest day, but the rest day is always Day 1 of the week. For example, if you load your plan to start on a Monday, the rest day is always on a Monday. If you load your plan to start on a Sunday, the rest day is on Sunday. Or, you can just move the rest day and adjust other workouts as described in the question above.