Yes u can refuse any days of work
You can pick your days off, and refuse to work on those days. I had to dodge calls always as there is always some shifts that needs emergency covering and you end up guilted into going. Stand your ground and work well in the meantime. They are very flexible though so that is good
You will not be required to work 7 days per week and you will be asked what you can work at interview stage. The role fits around your life also.