The motif of birth in a cave is . . . very ancient. This symbol is associated particularly with the winter solstice, when the sun has traveled to its farthest point away from the tilted earth and the light is in the nadir of the abyss. That is the date of the birth of the god Mithra, who is lord of light. He was born—we recall that his mother is the Earth—holding a rock-hewn weapon in his hand. Mithra was the principle competitor with Christianity, in the period of the first three centuries. The Christmas date was placed on December twenty-fifth, which was the solstice time, in order to compete with The Lord of Light, Mithra. No one really knows when Christ was born. It was settled on December twenty-fifth for mythological, not historical, reason.
Joseph Campbell, Thou Art That, p. 65