On the feast of the Epiphany of the Lord, my husband wrote with blessed chalk 20 + C + M + B + 12 above our front door. These are the initials of the Three Magi: Casper, Melchior and Balthasar. CMB is also an abbreviation for Christus mansionem benedicat, "May Christ bless the house."
According to the explanation given to us by the Benedictine monks at Monastery of Our Lady of the Cenacle here in Tulsa, Oklahoma, U.S.A., "the letters recall the day on which the inscription is made, as well as the purpose of blessing. The crosses represent the protection of the Precious Blood of Christ, Whose Sacred Name we invoke, and also the holiness of the Three Magi sanctified by their adoration of the Infant Christ."
Further, "the inscription is made above the front door, so that all who enter and depart this year may enjoy God's blessing...The inscription is made of chalk, a product of clay, which recalls the human nature taken by the Adorable and Eternal Word of God in the womb of the Virgin Mary, by the power of the Holy Spirit."
As instructed, we read the Prologue of Saint John's Gospel (1: 1-14), followed by the Our Father, and the following prayer:
OGod, who by the leading of a star didst manifest thy only begotten Son to the Gentiles: mercifully grant that we who know thee now by faith may be admitted to the vision of thy majesty. Through the same Christ our Lord. Amen.