Christians for many centuries would focus during the final week of Advent on expelling any devils that may be lurking in the corners of their house and property. They believed in a basic spiritual principle that in order to invite God into their home, they needed to "exorcise" it of any evil influence.
This was not a formal exorcism performed by a priest, but more of a "family" service of prayer, led by the head of the household.
Francis Xaver Weiser explains this Advent custom in his book Handbook of Christian Feasts and Customs.
It may seem like a superstitious practice to some, but it reflects an authentic Christian worldview. Demons do exist and they don't like it when we celebrate such days as Christ's birth. It would make sense that they would want to do anything in their power to disrupt such a celebration.
This custom recognizes this reality and invokes the power of God to drive the evil spirits away. God is always the one who does the work; we simply invite him to exercise his power.
