The beautiful Epiphany tide custom of chalking your door is coming up.

What’s that all about?

There is a terrific explanation of the tradition here 

Put simply, you use blessed chalk to inscribe on the lintel of the door of your home a simple formula which includes the numerals for the year in question and the letters C + M + B.

It looks like this: 20 * C + M + B * 20

So what do the letters “C, M and B” stand for? Most people will tell you that they stand for the names of the three wise men–Caspar, Melchior and Balthasar–and that is certainly a venerable tradition, but I think it is not the best tradition and one that doesn’t really make sense.

Instead the better explanation is that it stands for the Latin phrase Christus mansionem benedicat which means “May Christ Bless this Home.”