Epiphany inscription over the doorway of the home:
20 + C + M + B + 09
The letters have two meanings. They are the initials of the traditional names of the Three Magi: Caspar, Melchior and Balthasar. All three revered as saints and honored in the Munich cathedral. We abbreviate the Latin words "Christus mansionem benedicat" ("May Christ bless the house"). 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, whom we invoke, and the holiness of the Magi who adored of the infant Jesus. The inscription is made above the front door, so that all who enter and depart this year may enjoy God's blessing. The month of January bears the name of the Roman god Janus, the doorkeeper of heaven and protector of the beginning and end of things. This blessing "christens" the ancient Roman observance of the first month.
Blessing of Chalk
V. Our help is the name of the Lord.
R. Who made heaven and earth.
V. The Lord be with you.
R. And with your spirit.
Let us pray.
Bless, O Lord God, this creature chalk
to render it helpful to your people.
Grant that they who use it in faith
and with it inscribe upon the doors of their homes
the names of your saints, Caspar, Melchior, and Balthasar,
may through their merits and intercession
enjoy health of body and protection of soul.
Through Christ our Lord.
And the chalk is sprinkled with Holy Water.
Can a lay person pray this prayer and write the letters over the doors of the house or should it only be done by a priest?
Only a priest or deacon can bless people & things, e.g., chalk. Anyone can write inscription over the door with the formula: "May Christ bless the house," or using the Latin equivalent. The prayer of blessing of chalk is offered here just in case deacons/priests don't have it and/or the laity can take the prayer to the deacon/priest for them to impart the blessing.
Does this need to be re-done ever year at Epiphany or is one time blessing?
It is an annual blessing of chalk and home. I believe it is the custom to bless the home annually during Epiphany, at least that's what I know. Priests I know bless homes up to the week before Lent.
Is it going too far saying that this custom reminds me of the Jewish Mezuzah?
Mark, I think the blessing of chalk with the prescribed inscription over the door is similar to the tradition of the Mezuzah. While not the same by any stretch of the liturgical imagination, there are parallels in "sacramentality" between Jewish and Catholic liturgical practices.
Peace!