SaturnaliaSaturnalia (from the god Saturn) was the name the Romans gave to their holiday marking the Winter Solstice. Over the years, it expanded to a whole week, the 17th thru 23rd of December. It also degenerated from mostly tomfoolery, marked chiefly by having masters and servants switch places, to sometimes debauchery, so that the (lower case) word "saturnalia" came to mean "orgy."
The customary greeting for the occasion is, "Io, Saturnalia!" -- io (pronounced "yo") being a Latin interjection related to "ho" (as in "Ho, there").