Tomorrow is Buß-und Bettag according to my calendar. What is this? A web search only brings up pages in German.
It means “penance and prayer”. As far as I can gather, it was unique to the Church in Germany from rather early on, an annual day of repentance for sins. In the broader Church, it was replaced by more frequent confession. In the 16th century, Protestants enacted it as an official holiday and it was a legal holiday in at least some parts of Germany until the 20th century.
This site (in German) basically says that it is a very old kind of German ‘holiday’. It means Days of Penance and Prayer. According to that site, it started in Roman Catholicism, then spread after 1532 in Protestant areas. There used to be lots of different days. Only in 1934 a unified Day was picked. Until 1995 it was an official holiday in Germany.