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.
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1 Jan.  New Years Day

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

7 April  Good Friday  (Karfreitag)

9 April  Easter (Ostern)
10 April  Easter Monday

1 May  Labor Day

18 May   Ascension Day  (Christi Himmelfahrt)

28 May Pentecost (Pfingsten)
29 May Pentecost Monday  (Pfingstmontag)

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

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

3 Oct.  Unification Day (Tag der Deutschen Einheit)

1 Nov.  Allerheiligen   Baden-Württemberg, Bavaria, Rheinland Pfalz, Saarland, Nordrhein-Westfalen

17 Nov. Buß & Betag   Sachsen

26 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