By the beginning of December, writes Columella, the farmer should have finished his autumn planting (
De Re Rustica, III.14). Now, with the approach of the winter solstice (December 25 in the Julian calendar), Saturnus, the god of seed and sowing (Latin
satus) was honored with a festival.
The Saturnalia officially was celebrated on December 17 (XVI Kal. Jan.) and, in Cicero's time, lasted seven days (counting inclusively)—from December 17 to 23. Augustus limited the holiday to three days, so the civil courts would not have to be closed any longer than necessary (Macrobius,
Saturnalia, I.10.4), and Caligula extended it to five (Suetonius,
Life of Caligula, XVII; Dio,
Roman History, LIX.6.4), the fifth day restored by Claudius after it had been abolished at one time (
Roman History, LX.25.
. Still, everyone seems to have continued to celebrate for a full week, extended, says Macrobius, by celebration of the Sigillaria on the last day of Saturnalia, so named for the small earthenware figurines (
sigillaria) that were sold then (
Saturnalia, I.10.24).
http://penelope.uchicago.edu/~grout/encyclopaedia_romana/calendar/saturnalia.html