K. A. Heinrich Kellner, Heortology (1908)
“ It is only natural that the winter solstice should give rise to a festival, and find its place marked in the Calendar of Feasts. Indeed, in the Roman Calendar of much later date—that of Philocalus—the 25th December is marked as the birth-day of the unconquerable Sun-God (Natalis Solis Invicti) .Since on the 21st December the sun reaches its lowest point, and then begins once more to rise higher in the heavens, man, in his simplicity, marked the day on which this change in the sun became perceptible as the new birth or birth-day of the sun, the invincible Sun-God. ”
