The Pantheon still has the largest unreinforced concrete dome in the world, which every day at noon is illuminated by a Circle of light that is located exactly on the side of the entrance portal.
The Sun has a different height depending on the season: it is higher in summer, when the days are longer, and lower in winter, when the days are shorter.
So the Circle of light illuminates the dome at a different height depending on the season, transforming it into an enormous spherical sundial that marks the passage of Time.
The shortest day of the year, December 21, corresponds to the Winter Solstice, which was very important to ancient peoples: it marked the moment of the year when it seemed that the Sun was struggling to rise again and there was a fear that the end of the world was coming.
The Romans exorcised those fears with the feast of the Saturnalia, which were a rite of passage from the old year to the new year, quite similar to our Christmas and New Year's celebrations. Processions and sacrifices were held in honor of the god Saturn (later replaced by Dionysus), who regulated the cyclical flow of Time; and then there were banquets with night vigils waiting for dawn, exchange of gifts and celebrations.
It should be remembered that in the ancient world Time was not endlessly flowing forward as it is now: but was cyclical and circular, it always repeated itself in the same way. The year began with the Winter Solstice, then came Spring with the gradual awakening of Nature, and culminated at the Summer Solstice, which was the time of maximum flourishing. Then a descending curve began, with the arrival of Autumn, the end of the year and the beginning of a new cycle.
Starting with Augustus, the priestly college of the Arvales celebrated rituals linked to imperial power. On January 3rd they made sacrifices to propitiate the health of the emperor and the happiness and safety of the empire; they dissolved the vows of the previous year and made others for the new year.
Those ceremonies probably took place also in the Pantheon and survived with the advent of Christianity. The Pantheon became in fact one of the "stationary" churches of Rome (chiese stazionali), where on January 1st of each year the Pope led a solemn procession; two other processions took place on Good Friday and on May 13th, the day of the consecration.
In the winter months the illuminations of the Pantheon are not as spectacular as the Arch and the Square of Light, which occurr in April and September, but they give an idea of the passage of Time. Their symbolic meaning is explained in detail in the book by Marina De Franceschini and Giuseppe Veneziano «Pantheon. Architecture and Light», with splendid unpublished photographs.