Shop opening hours can greatly vary, although, so far, shops across Germany are not allowed to open on Sunday or on (regional) bank holidays. In the more conservative southern state of Bavaria, shops are allowed to be open between 6 a.m. and 8 p.m., while in Saxony, people can (theoretically) grocery shop until 10 p.m.; that shopkeepers can keep their stores open around the clock in many other German states feels alien to me! If I want to buy toiletries or something to eat after 8 p.m., I need to head for the Munich central terminal or the nearest gas station.
On the other hand, in some rural and neglected areas, especially in the east, people do not have any shops to go to any more, let alone train stations, post offices, or hospitals!
Remember that Saturday is not a working day in Germany; many companies also close early on Fridays. While I, as a freelancer, sometimes meet foreign visitors on a Saturday, only on very rare occasions do I accept any meetings for Sundays. Sunday is sacred to (most) Germans, irrespective of whether or not they go to church.