A venerable custom of the Church on this Epiphany weekend is to bless one's home.  Chalk is blessed at church and distributed to the faithful... who then recite the following prayer at home:

"Bless O Lord, almighty God this home that it be the shelter of health, chastity, self-conquest, humility, goodness mildness, obedience to the commandments, and thanksgiving to the Trinity.  May blessing remain for all time on this dwelling and those that dwell herein."

Using the chalk, you can then inscribe on the lintel of your door the formula, "20+C+M+B+23"

The formula has two meanings. First, the initials of the Magi — Caspar, Malchior, and Balthazar — who came to visit Jesus in His first home. It also abbreviates the Latin phrase, Christus mansionem benedicat: “May Christ bless the house.” The “+” signs represent the cross, and the “20” at the beginning and the “23” at the end mark the year.