It began as a farmer's festival and blossomed into an everyone festival to honor Saturn, who was, according to Wikipedia, "a god of generation, dissolution, plenty, wealth, agriculture, periodic renewal and liberation. In later developments, he also came to be a god of time."
Now, is it one day, three days, a week? Well, according to the blog Following Hadrian, it was originally "celebrated on a single day, on the fourteenth before the Kalends of January (December 19), but it was later extended to three days. With the Julian reform of the calendar, Saturnalia was celebrated sixteen days before the Kalends of January (December 17). However, by the end of the Republic the festival was so popular that it expanded to cover a week. The emperor Augustus would shorten it to a three-day holiday during his reign but Caligula later extended it to five days. According to the author Macrobius, the celebration of Saturnalia was extended with the on the 10th day before the KalendsDecember 23) so named for the small terracotta figurines which were sold in Roman shops and given as gifts to children."
This was only one of many pagan celebrations that called for bringing in greenery and decorating with berries to give the home a festive air in the dead days of Winter. People would dress up in festive outfits and give gifts to their children. There's nothing new about this.
In fact, there have been festivals of the Sun for thousands of years, and some have fallen on December 25th. It's no accident, of course, that Christmas day appropriated a pagan holy day. Christians were always doing that. It was a quick and dirty way to convert the locals. Just tell them that well they always thought was holy is now called Saint Bridget's Well, or Saint Mary's Well. And if you're already celebrating a day of the Sun, why not celebrate a "real" god on his special day.
So, death and renewal, festivities and failures, greenery and debauchery. What a festive time of year! Merry Everything!