"time of merrymaking," 1591, from L. Saturnalia, ancient Roman festival of Saturn (held in December), a time of merrymaking for all, from neut. pl. of adj. Saturnalis "pertaining to Saturn," from Saturnus (see Saturn). The extended sense of "period of unrestrained revelry" is first attested 1782.
[sat-er-ney-lee-uh, -neyl-yuh] 
) the festival of Saturn, celebrated in December in ancient Rome as a time of unrestrained merrymaking.