Chalking the Door

The following is from Wikipedia because I can't explain it any better:

Either on Twelfth Night (5 January), the twelfth day of Christmastide and eve of the feast of the Epiphany, or on Epiphany Day (6 January) itself, many Christians (including Anglicans, Lutherans, Methodists, Presbyterians and Roman Catholics, among others) chalk their doors with a pattern such as 20 ✝ C ✝ M ✝ B ✝ 21, with the numbers referring "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."^[1] Another form, for Three Kings day, is to mark the door with IIIK (the Roman numeral three followed by "K" for "Kings").