The third century AD was a terrible time for the Empire, with a succession of generals usurping the Imperial crown and the empire assaulted by external enemies like the great Persian king Sharpur II. Things got so bad that the Empire split into three pieces - a "Gallic Empire" in the West comprising Britain, France, and Spain; the rich eastern provinces of Egypt and Syria falling under the domination of Queen Zenobia's oasis city state of Palmyra, and a rump Empire of Italy and Africa. It was really possible for a moment that the Roman Empire would simply dissolve - the bonds holding it together looked too weak to hold.
|A gold coin from Aurelian's reign|
So in five short years Aurelian restored the Roman world. But he wasn't just one of the best generals in Roman history, he was also a great statesman. He turned his mind to why the Empire was so fragile; if he could knit it more tightly together he might be able to prevent a repeat breakup. Aurelian believed that a big problem was that the Empire was a collection of diverse peoples - Gauls and Britons and Egyptians and Syrians who all had different cultures and beliefs. In short, they had little in common other than the Emperor of the day and everyone had just seen how that had worked out.
And so Aurelian tried to overlay some commonality on his peoples. Each worshiped their own local gods, but most of these religious systems were fairly flexible. Aurelian introduced an Empire-wide cult, thinking that having some similarities would help create a common sense of Roman-ness. Aurelian chose a cult that was popular with the Army since the closest thing that the Empire had to a single common institution throughout the Empire was the Army.
Sol Invictus was popular with the troops, the Unconquered Sun god. Most parts of the Empire adopted this seamlessly as one of the many gods, although it seems that Aurelian seemed to believe that Sol Invictus was the only god who took many forms which were interpreted as the local deities. This was an emergent idea in the Ancient world and an expression in the chronicles say the one wax takes many moulds.
Aurelian introduced his cult on December 25, 274 AD and it became really the first Empire-wide holiday. He succeeded in founding a common belief across the Empire, perhaps succeeded more than even he hoped. Because the idea stuck: Emperor Constantine didn't just introduce Christianity. It's from him that we get the word Sunday, since he decreed that across the Empire the weekly day of rest would be the day of rest - the dies Solis.
And so the early Church had a challenge from a popular cult, but this was also an opportunity for them. Sol Invictus was the first half step towards monotheism and identifying Jesus Christ with the unconquered sun didn't actually turn out to be all that hard for the early Church Fathers. Indeed, what is Easter if not the celebration of the Unconquered Son? December 25 stuck in the calendar. It's been celebrated all the way down through the ages - ever since 274 AD.
It wasn't the silliness of Saturnalia that had to be co-opted, it was the Feast of the Nativity of the Unconquered Son. May tomorrow's feast day be festive indeed. You might even want to offer a toast to Aurelian Restitutor Orbis.