By law, "the Sundays and the public holidays remain protected as days of rest from work and of spiritual elevation" (Art.
Public holidays apart from the Sundays (there must be some of them constitutionally) can be declared by law either by the Federation or by the Länder for their respective jurisdictions.
A couple of days are designated as stille Tage ("quiet days") by state legislation, which regularly means that public dancing or sport events, music at inns (if live or if not much quieter than usual) etc.
Frequently flags are ordered ad hoc to be shown at half-mast in cases of national mourning.
Also, Christmas Eve is developing into a semi-holiday: from mid-afternoon it is practically treated as a holiday, and while shops still open in the morning, for other businesses (apart from those that work even on holidays) this is becoming increasingly unusual; schools are closed in any case.