On or near the feast of the Epiphany, the tradition has been to ask God’s blessing on homes and mark the door post or lintel with chalks that have been blessed for that purpose.
The doors are chalked with the legendary names or initials of the three Magi, and the numerals of the New Year, connected with a series of crosses.
The initials C, M, and B commemorate the Magi, Caspar, Melchior, and Balthasar, but also stand for the Latin prayer-request Christus Mansionem Benedicat: “May Christ bless this house.” In this way, the door of a house would be marked as follows: 20+C+M+B+19

Updated facebook profile picture below. (Taken at the end of my minor ministry mass of Acolyte.)