January is named after the two faced Roaman god Janus who was known to have tow faces, one looking at the past and one looking towards the future. Janus was also a guardian of thresholds such as doorways and crossroaads. One of Janus symbols are keys and key holds which open doorways which are considered important thresholds being between the outside and the inside.
During the various winter solstice festials door were decorated with evergreens which were connected to the rebirth of the new year.
Such things of gargolye like door knockers were used to scare away negative forces. such practices of nailing horse shoes, notably iron horses are employed for much the same reason.
Religious artifacts such as crosses could be placed over the doors for a similar reason. And we all know that vampires cannot enter a home without being invited, so such artifacts as corsses and horse shoes wok as abane against such evil doers such as ghosts and demons. And so, it goes.