When it is an official holiday in Germany, count on all of the stores being closed. Restaurants, cafes, tourist attractions, souvenir stores, ice skating rinks, pools, movies, are usually still open, but it depends on the holiday. Most museums will be open too, but it is always wise to check their websites. Some holidays cause more closures, like New Years, Christmas, or Good Friday.
Emergency shopping can be done at the main train station in Frankfurt or the airport. There you will find groceries, pharmacies, and bakeries open.
*****************************************************

1 Jan.  New Years Day

6 Jan.  3 Kings Day, (Heilige Drei Könige) Bavaria, Sachsen-Anhalt, Baden-Württemberg

8 March  International Frauen Tag  Berlin, Mecklenburg-Vorpommern

18 April Good Friday  (Karfreitag)

20 April  Easter (Ostern)
21 April  Easter Monday

1 May  Labor Day

29  May   Ascension Day  (Christi Himmelfahrt)

8 June Pentecost (Pfingsten)
9 JunePentecost Monday  (Pfingstmontag)

19 June Corpus Christi Day  (Fronleichnam) Bavaria, Hessen, Rheinland Pfalz, Saarland, Baden Württemburg, Nordrhein Westfalen

15 Aug.  Ascension of Mary  (Maria Himmelfahrt) Bavaria & Saarland (only parts)

3 Oct.  Unification Day (Tag der Deutschen Einheit)

31 Oct. Reformation Day   Brandenburg, Bremen, Hamburg, Mecklenburg-Vorpommern, Niedersachsen, Sachsen, Sachsen-Anhalt, Schleswig-Holstein,Thüringen

1 Nov.  All Saints Day (Allerheiligen)   Baden-Württemberg, Bavaria, Rheinland Pfalz, Saarland, Nordrhein-Westfalen

19 Nov. Buß & Betag   Sachsen

23 Nov.  Day of the Dead – Toten Sonntag (not a holiday, but festivities are limited, Christmas markets are usually closed)

24 Dec. Christmas Eve – not a holiday, but stores and almost everything closes by 14:00
25 Dec.  Christmas
26 Dec. 2nd Christmas Day

31 Dec. New Years Eve – not a holiday, but stores and almost everything closes by 1400