rev peter writes

Either on Twelfth Night, the twelfth day of Christmastide and eve of the feast of the Epiphany, or on Epiphany Day itself, many Christians (including Anglicans, Lutherans, Methodists, Presbyterians and Roman Catholics, among others) will chalk their doors with a pattern such as 20+C+M+B+21.

The numbers refer to the calendar year 20 and 21, for instance, for this year, 2021. The crosses stand for Christ; and the letters have a two-fold significance. C, M and B are the initials for the traditional names of the Magi, Caspar, Melchior and Balthasar, but they are also an abbreviation of the Latin blessing "Christus mansionem benedicat," which means "may Christ bless this house."

 In some localities, but not in all, the chalk used to write the Epiphany tide pattern is blessed by a Christian priest or minister on Epiphany Day; Christians then take the chalk home and use it to write the pattern. This Christian custom of chalking the door has a biblical precedent as the Israelites in the Old Testament marked their doors in order to be saved from death; likewise, the Epiphany practice serves to protect Christian homes from evil spirits until the next Epiphany Day, at which time the custom is repeated.

 Families also perform this act because it represents the hospitality of the Holy Family to the Magi and all Gentiles. It thus serves as a house blessing to invite the presence of God in one's home, and at the Epiphany meal, an extra place will often be set to symbolise the willingness of the family to offer hospitality to the uninvited guests, to the stranger.