Saturnalia is the ancient Roman holiday celebrated around the winter solstice.
It was in praise of the god Saturn, and consisted of a huge drunken orgy.
Another common practice during this festival was the switching of slave and master roles. for one day, all the slave owners gave up their power to their slaves.
the common saying for this festival (ex. merry Christmas) was "Io Saturnalia" pronounced "yo saturnalia."
"hurray, for it is the solstice! Io Saturnalia everyone. Now fetch me some wine and let us get naked."