Stores are open on Saturdays, at least until 5 r 6 pm. Larger business (downtown) are open until 8pm.
Stores are not open on Sundays (maybe a couple of times a year only), but restaurants are.
Bavaria is the most religious (catholic) state in Germany, with the most holidays. They observe Good Friday and the Monday after Easter as well (Ostermontag).

AFAIK, the Frauenkirche does have a choir. Here's the web site (but the calendar only goes until March 20th).
http://www.muenchner-dom.de/index.php?id=53