Epiphany. The Day of the Three Kings is always on January 6 and today is a holiday in Germany.
Children dress up as the three kings who visited Jesus in the manger in Bethlehem. The kids knock on doors and offer to write a symbol of blessing over the top of people entry-way doors. In exchange, the “blesee” is expected to give a small donation to the Sternsinger, an organization that does various charitable works around the world.
The formula is quite simple: the 20 and the 10 on either end signify the current year. The C+M+B stand for the names of the 3 wise men, Caspar, Melchior, Balthasar. I have also heard that C M B stands for the latin, “Christus Mansionem Benedicat” – basically, God Bless this House.
From the Evangelisch (Protestant) Church (German)