Every year August 15th is the christian holiday Maria Himmelfahrt.
In Munich the stores are closed.
Mariä Himmelfahrt is a holiday in catholic communities.
In protestant it isn´t a holiday. For example in Nueremberg it is a regular working day.
Grocery shopping – supermarkets
There are some supermarkets open on sundays and holidays in munich
Rewe to go at Main Station (Hauptbahnhof)
Edeka at Main Station (Hauptbahnhof)
Edeka at East Station (Ostbahnhof)
I hope this helps!
I wish you a great stay in munich!