Janus instituted the Saturnalia as a yearly tribute to his friend, Saturn. For mortals, the festival provided a yearly symbolic return to the Golden Age. It was an offense during this period to punish a criminal or start a war. The meal normally prepared only for the masters was prepared and served first to the slaves, and in further reversal of the normal order, it was served to the slaves by the masters. All people were equal and, because Saturn ruled before the current cosmic order, Misrule, with its lord (Saturnalia Princeps), was the order of the day.
Children and adults exchanged gifts, but the adult exchange became so great a problem -- the rich getting richer and the poor getting poorer -- that a law was enacted making it legal only for richer people to give them to poorer.
According to Macrobius' Saturnalia, the holiday was originally probably only one day, although he notes an Atellan playwright, Novius, described it as being seven days. With Caesar's changing of the calendar, the number of days of the festival increased.