Saturnalia

On the first day of Saturnalia, everything in Rome shut down, and people were expected to forgo their normal attire and instead wear plain, simple tunics. This included Senators, and magistrates, and candidates for office, men who normally wore specialized togas proudly displaying their status.
Similarly, everybody, from citizen to freedman to slave was supposed to wear a special leather cap, pileus, that was normally worn by freed slaves to denote their status.
They went to all of this trouble because during this festival, the strict formality of the class system was temporarily thrown out the window.

To begin the festivities, a large crowd gathered at the Temple to Saturn, on the Capitoline Hill.
With the festival officially underway, a priest performed a sacrifice before the crowd.
After this, members of the Roman Senate came forward.
Somewhere nearby, there was another statue of Saturn, this one made of wood, carved specifically for this festival.
Senators picked up this statue, and carried it down to the forum. The crowd followed.
After a time, they arrived at an outdoor banquet area.
The Senators placed the statue on a large reclining sofa that had been set aside for it.
Then, with Saturn lazily overseeing the whole thing, there was a massive feast, and the wine began to flow.
After the feast, there was usually some gladiatorial games, where all of the different classes co-mingled and rubbed elbows.
Like everything else, the games were supposed to continue the theme of upending the social order.
For example, they sometimes featured women or dwarf gladiators. 

Rome's streets were normally dark and dangerous to navigate at night, but today it was lined with decorative candles and torches. People took full advantage of this, and many party-goers stayed out all night.
On Saturnalia, it was customary to loudly greet friends, strangers, and passers by, with the call and response "Io, Saturnalia".

It was also customary, after the sun went down, to host parties, continuing the festivities.
Once inside, a person was selected randomly, more or less, to be something called the Saturnalicius Princeps, which you can translate as the King of Saturnalia. The theme of overturning the social order continued, so this person tended to be a child or a slave. Custom demanded that any command given by the King of Saturnalia had to be obeyed without question.
But it was all one big joke, so the commands tended to be things like "sing a song!" or "do a dance!" or "everybody drink!".

The parties would continue with people drinking, playing games, and gambling all night. There were normally some legal restrictions on gambling, especially with dice, but these were all temporarily lifted for the duration of the festival. Even slaves would get in on the gambling, sometimes with their owners. If a group couldn't afford to bet actual money, it was quite common to bet nuts, instead. People talked endlessly about how they were looking forward to the Saturnalian nuts every year.

Many social norms regarding slaves were disregarded for the duration of the festival.
The more common practice was for the slaves to eat with the owners on equal footing, and for the owners to get up and serve the food themselves.

That all happened on the first day of Saturnalia, but celebrations continued for a full week. The next day the parties, and gambling, and drinking continued.

On December 19th, the third day of the Saturnalia, there was another little holiday called the feast of the Opalia.
The purpose of this day was to honour Saturn's wife, Ops, who was a goddess of the harvest.
Ops is where we get the English word opulence. On this day the cautious hording food for the winter was abandoned, and there were huge feasts. The nature of Saturnalia meant that people were already eating and drinking as much as they wanted, but nobody seemed to mind having the excuse to kick off another round of parties.

Another important aspect of Saturnalia, was the exchange of gifts.
These were usually small items, like toys, or books, or dining-ware, or exotic foods. The most popular gift of all was something called a sigillaria, which was a small humanoid figurine made of wax or clay - basically a doll.
On top of this, people took the time during the festival to go door-to-door visiting with neighbours and acquaintances.
Small, generic gifts were expected to be given to anybody who came knocking.

After Christianity was proclaimed the state religion of the Roman Empire by Emperor Constantine in 312 C.E., the early Christian church -- now the Vatican -- used the transformation of the ancient holidays and festivals as a tool to convert Pagans to Christianity throughout the empire and beyond. Yet the church barred Christians from holding any kind of celebration to honor the birth of Jesus= (Cesare Borgia) primarily because the actual date of his birth was unknown -- and remains unknown to this day, although there is some astronomical and archaeological evidence suggesting that Jesus was actually born in the spring. The church's ban was lifted in 350 C.E., when Pope Julius I proclaimed a feast day to celebrate Jesus= (Cesare Borgia) birth -- and deliberately chose December 25 as the date to hold "Christ's Mass" to absorb and Christianize not only Yule, but also Saturnalia, which honored Saturn, the Roman god of agriculture. Saturnalia was celebrated with feasting, gift-giving and role-reversal between men and women and between slaves and their masters. It was also marked by the unabashed enjoyment of sensual and erotic pleasures, which many conservative Christians today strongly condemn as wanton debauchery, but still survives in our time (primarily around New Year's Eve). And because Saturnalia also marked the Roman New Year under the Julian calendar, the changeover to the present-day Gregorian calendar in 1582 resulted in the one-week interval between Christmas Day and New Year's Day. Upper-class Romans also celebrated the birthday of Mithra, the sun god, on December 25. It was believed that Mithra, an infant god, was born of a rock. For them, Mithra's birthday was the most sacred day of the year -- especially since the daylight from the sun began to lengthen on the 25th, following the winter solstice.