The chalk over the door, and the rite behind it.
On Epiphany, mark the lintel in blessed chalk with the year and the letters, for example 20 + C + M + B + 26.
The letters stand for Christus mansionem benedicat, may Christ bless this house, and are also read as the traditional names of the magi.
A priest may bless a home at any time, and it is usual to ask when you move in.
A blessing asks God to claim a place. Catholics do not think of a house as neutral ground, and marking the door is the same instinct as the blood on the doorposts in Exodus.
House blessing at Epiphany is a northern European custom that spread widely. The chalk is blessed at Mass and taken home by the family.
The rite itself is much older and appears in the Church's books of blessings as one of the ordinary services a priest performs for a family.
Free. The chalk marking may be done by any member of the household with blessed chalk.