The celebration of Sol Invictus was not limited to just one specific holiday, but was a conflation of different pagan rituals celebrated for a long time. There is no specific pamphlet found in history, but many believe it was adapted and celebrated by different people at different times.
This transcript discusses how Christianity is intertwined with Paganism through blood sacrifice, multiple gods, and other strange rituals. The fourth century church turned Jesus' birthday into a celebration on the winter solstice, which aligns with pagan fertility rites.
The origins of Christmas can be traced back to the Roman holiday of Sol Invictus, the unconquered sun, which was the official religion of Rome before Christianity took over. While there may be arguments over which traditions Christmas celebrators took from other religions, it is clear that many aspects of the holiday have pagan roots.
Saturnalia was an ancient Roman festival that celebrated the god of agriculture, Saturn, and included a week of role reversals, gift-giving, and partying. Its exact origins are unknown, but historians believe it was celebrated for centuries and the traditions evolved over time.