The Gospel tells us that the Magi found Christ "on entering the house."
The door to your home is a holy threshold. You can bless those who come in and go by inscribing above the door in chalk the first 2 numbers of the year + C + M + B + the last 2 numbers of the year.
So for this year you would write 20CMB10.
Tradition tells us that the C,M and B stand for the names of the Magi: Caspar, Melchior and Balthasar.
It may also stand for Christus mansionem benedictat which means "may Christ bless this dwelling"
It's appropriate to bless your door in January - janua means 'door' - and January is the door to the new year.
After inscribing the doorway, say this prayer:
God of Bethlehem and Cana, God of Jordan's leaping waters, in baptism You bring us into Your family. You wed us and embrace us as Your beloved. May we fill this place with kindness to one another, with hospitality to guests, and with abundant care for every stranger. By the gentle light of a star, guide home all who seek You on paths of faith, hope and love. Then we will join the angels in proclaiming your praise: Glory in heaven and peace on earth, now and for ever. Amen.
Some then sing a Christmas carol.
(Thanks to the Archdiocese of Chicago for this information)