Epiphany or DreiKoinigsFest is the 13 day after Christmas (or after the 12 days of Christmas).
Apparently the chalk markings mean either or both of the above.
From German and German/American Traditions of Holidays
On the evening before Three Kings, traditionally there were prayers, blessed dried herbs would be burnt and their aromatic smell would fill the house. Doorways would be sprinkled with holy water and the master of the house would write with chalk C + M + B and the year above the house and barn door and say: "Caspar, Melchior, Balthasar, behütet uns auch für dieses Jahr, vor Feuer und vor Wassergefahr." ("CMB, protect us again this year from the dangers of fire and water.") C + M + B has traditionally been translated with Caspar, Melchior and Balthasar, however, according to the Church it stands for "Christus Mansionem Benedictat" (Christ bless this home).