The solstice is the exact moment of change when darkness recedes and light continually shines longer. The moment of the solstice occurs when Thor hurls a bolt of lightning through the black night storm and strikes an oak tree. In that moment light and dark are simultaneously both powerful. The moment marks the shift where the power of light now takes dominion. Dark is good because it holds the beginning new life.
Light is good because it brings birth.
Such tremendous goodness requires a tremendous celebration. The flash of lightning, bright and hot, creates mistletoe. Mistletoe, called allheal, a bestower of life and fertility, a protection against poison and a provider of safe conduct through the underworld, is of the dark, fertile, gestating side of mid winter. Born in fire, mistletoe puts out fire and keeps the dark safe. Hung on doorways of homes an barns it keeps evil spirits and witches away.