Mistletoe and holly, Yule-log and Yule-candle, belong to the same category.
The mistletoe was regarded by the Druids with religious veneration, and its berries of pearl, as symbolic of purity, were associated by them with the rites of marriage. From this the transition was but slight to the lover’s kiss beneath its mystic bough during the Christmas-tide.
At this festive season also they kindle bonfires upon the hill-tops. Nor must we forget that our pagan progenitors burned a great log and a mammoth candle upon the 21st of December, which, being the shortest day in the year, was regarded as the turning-point in the conflict between the contending forces of winter and spring. Advent is the herald of Christmas. In Protestant as well as Catholic countries choristers and school-boys duringithe “holynights” go from house to house singing songs or Christmas carols, with which to usher in the auspicious day.
In the south of Germany they accompany the singing by knocking at the doors with a little hammer, or throwing pease, beans, or lentils at the windows. Hence the origin of the name of “knocking nights.”
Source: Harper’s Magazine, 1873