Chalking the door

Chalking the door is one of the Christian Epiphanytide traditions used to bless one's home, as well as a Scottish custom of landlord and tenant law.
Epiphanytide.
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) write on their doors with chalk in 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 Balthazar), but they are also an abbreviation of the Latin blessing "Christus mansionem benedicat", which means, "May Christ bless this house"." Another form, for Three Kings day, is to mark the door with IIIK (the Roman numeral three followed by "K" for "Kings"). Though chalking the door is done most commonly on Epiphany Day itself, it can be done on any day of the Epiphany season.
In some localities, but not in all, the chalk used to write the Epiphanytide 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 Epiphanytide 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.
Scotland: A historical method to notify tenants of eviction.
In Scotland through the mid- to late 1800s, chalking the primary door of a tenement was a way of notifying residents that they must move from their residence by a given day.
By custom, leases and other similar contracts began or ended on the Scottish Term Days, which are Whitsunday and Martinmas. If a landlord wished to evict tenants in a particular tenement, the law dictated that, at the landlord's request, a burgh officer, in presence of witnesses, would chalk the primary door of the tenement forty days before Whitsunday or St. Martin's Day. The burgh officer would then record the fact that the chalking had been accomplished and this document was signed by the officer and the witnesses.
The chalking indicated to tenants that they must vacate the premises by the next Term Day, when the lease expired. Tenants who failed to vacate by that date could then be evicted on six days' notice via a so-called "charge".