In The Golden Bough, Sir James Frazer describes early Europeans building solstice fires at year's end to strengthen the sun.
Seeing the sun steadily weakening, steadily falling in its arc across the sky, they did what they could to restore it to health.
Birds and other creatures of the natural world respond to the length of winter days. Much of the rhythm and timing of birds' behavior, including migration and breeding, is determined by the length and intensity of the sun's light.