Saturnalia was the big year-end celebration for the ancient Romans. This is not to mean it was only celebrated by senior Romans, but everyone back then. Why it wasn't called SaturnJanusalia is a good question, since both Janus and Saturn were responsible for time and change. But apparently Saturn was better at picking out decorations and wine choices. The whole point of the holiday was one of utter drunken debauchery, so nothing really has changed today.
The Christian church decided that they could stomp out Saturnalia celebrations by putting Jesus's birth (Christmas) in the same spot on the calendar. As you can easily see, it didn't work. Plus He is still distressed that He doesn't get birthday presents on the proper day. People were still hung over by the New Year, putting up dead trees in their houses and wearing Santa hats while giving gifts to each other including VD but not to Jesus. Lucky Jesus. Out of this confused mess, the Japanese have decided that Saturnalia/Christmas is also a big retail time and something to celebrate with KFC. WTF?