That was an ancient Persian/pagan/Roman feast held on December 25th.
Wikipedia is almost shockingly frank in it's coordination of "Christmas" with it's pagan roots.
An interesting note for those who still believe in the "christian Constantine" myth:
Constantine decreed (March 7, 321) dies Solis—day of the sun, "Sunday"—as the Roman day of rest [CJ3.12.2]: "On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed..."
Christmas was promoted in the Christian East as part of the revival of Catholicism following the death of the pro-Arian Emperor Valens at the Battle of Adrianople in 378. The feast was introduced to Constantinople in 379, and to Antioch in about 380. The feast disappeared after Gregory of Nazianzus resigned as bishop in 381, although it was reintroduced by John Chrysostom in about 400.
The Twelve Days of Christmas end on January 5. December 26 is St. Stephen's Day and January 6 is Feast of Epiphany This period encompasses the major feasts surrounding the birth of Christ. In the Latin Rite, one week after Christmas Day, January 1, has traditionally been the celebration the Feast of the Naming and Circumcision of Christ, but since Vatican II, this feast has been celebrated as the Solemnity of Mary, Mother of God.
In Colonial America, the Puritans of New England disapproved of Christmas. Celebration was outlawed in Boston from 1659 to 1681.
So, in case you felt like celebrating.....think twice about what it means.
Some links for Saturnalia:
An almost satanic description of how to celebrate Saturnalia.
Specifications on ancient decorations, gift giving, etc, (very similar to Solis Invicti)