Saturnalia was the ancient Roman festival held in honor of Saturn that influenced later winter holiday traditions.
The festival began on December 17 and was later extended across several days. Public religious rites, feasting, gift-giving, greenery, and temporary reversals of social roles were among its features. The exact practices changed during Roman history, so Saturnalia was not a single unchanging celebration.
Saturnalia took place near the winter solstice and before the modern Gregorian calendar. It was therefore a Roman midwinter festival, not a New Year's Eve celebration in the modern sense. Still, its customs are often discussed when historians compare ancient and later winter festivities.
The festival should not be confused with Kalends of January, the Roman celebration associated with the start of the new year. Similar seasonal customs can have multiple historical sources, so claims that one festival directly “invented” all later traditions are usually too simple.