German Federal Holiday.
Jan. 6, is Dreikoenigstag, or Three Kings Day, which marks the end of the 12 days of Christmas. During this week kids dressed as the three kings—Caspar, Melchior and Balthasar—go house to house, singing songs and collecting donations for an official church charity. They are usually accompanied by a fourth child, who carries a star, and the whole group is called the Sternsinger, or star singers. At the end of the singers' visit, they ask to mark/chalk your door.
The numbers 20 and 26 reflect the year, the asterisk stands for the star of Bethlehem and the three crosses are for the Holy Trinity. C, M and B is for "Christus Mansionem Benedicat," which means "God bless this house."
As a reminder, today is a German holiday here in Bavaria. Please expect limited services on-post and the closure of off-post shops.