In Roman times, the most important feast of the year were the Saturnalia, which coincided with the days of the Winter Solstice (December 21st), when the Sun seemed to struggle to rise again and the apparent death of Nature had to be exorcised with special rituals so that the Sun would resume its course.
Saturnalia were a rite of passage from the Old Year to the New Year, very similar to our Christmas and New Year's celebrations. Gifts were exchanged, and a great banquet was prepared to enliven the long night vigil, awaiting the rising of the new year's Sun. A game very similar to today's bingo was also played: the numbers drawn also had oracular functions and were used to predict the future.
It should be remembered that for the ancients, time was not in constant flux as it is today, but was cyclical and circular: it began with the Winter Solstice (December 21st), continued with an ascending parabola and the slow rebirth of Nature until spring after the spring Equinox (March 21st).
It then continued until the Summer Solstice (June 21st), which marked Nature's peak bloom and the harvest season. Then the descending parabola began until the autumnal Equinox (September 23rd), the time of sowing, and then went back to the starting point with the Winter Solstice on December 21st.
Many Roman buildings were oriented to create illuminations (hierophanies) precisely on the days of the Winter Solstice. We discovered this in Hadrian's Villa in Tivoli, in the buildings of Roccabruna and Accademia.
In the Accademia, at dawn on the Winter Solstice the sun enters through a door and its rays illuminate the building for all its length, passing through the Temple of Apollo, at dawn on the Winter Solstice.
At sunset on the Summer Solstice, the path of the sun's rays reverses 180° and enters through the door on the opposite side, to the north.
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wIn Roccabruna, the same thing happened in the Tempietto on the upper floor (now destroyed).
At dawn on the Winter Solstice, the sun entered through the main door, located at the top of a staircase.
This coincided with the moment when the temple doors were opened to "awaken" and dress the goddess Isis. The Tempietto must have resembled the one seen in a fresco in Herculaneum.
In the domed room on the lower floor of Roccabruna, during the Winter Solstice, a special light conduit still creates a Circle of Light inside the dome.
During the Summer Solstice, at sunset, the most spectacular illumination is seen, with a Blade of Light created by the light conduit at the center of the main façade.
The illuminations were a sacred luminous signal: those were the right days to celebrate rituals related to the Saturnalia and Fors Fortuna, as we explain in detail in our book «Villa Adriana. Architettura Celeste. I Segreti dei Solstizi»(italian edition only) which you can purchase on the Rirella Editrice website.
Originally, the Saturnalia were dedicated to the god Saturn. When he was replaced by Dionysus, the old rituals were supplemented by the wild celebrations and abundant libations typical of his cult. Shops, schools, and courts were closed, and wars were suspended.
In the 3rd century AD, the Saturnalia was superseded by the cult of Sol Invictus, a solar deity of Eastern origin imported briefly in 218 AD by Emperor Elagabalus. In 274 AD, the cult of Sol Invictus was established.
Emperor Aurelian made it the official State cult. The Sun god was Dominus Populi Romani, and Emperor Aurelian became Dominus et Deus, deified during his lifetime as a descendant of the Sun god by birth.
The Sol Invictus was the invincible Sun, reborn every day and every year to a new life and vanquishing darkness and Death.
Emperor Hadrian was depicted as Sol Invictus driving the Quadriga of the Sun, a bronze sculpture that appears to have atop his Mausoleum, now the Castel Sant'Angelo in Rome, as we explain in our book «Castel Sant'Angelo. Mausoleum of Hadrian. Architecture and Light» (also Englsh edition), which you can purchase on the Rirella Editrice website.