(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 through 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 "oy") being a Latin interjection related to "ho" (as in "Ho, there").
Other Roman festivals and rites include the Ambarvalia and the Lupercalia.
It has been postulated that Christians in the fourth century assigned December 25th as Christ's birthday (and thus Christmas) because pagans already observed this day as a holiday. This would sidestep the problem of eliminating an already popular holiday while Christianizing the population.