Short answers, pulled from the story.
Saturnalia reflected a mythical Golden Age where humans enjoyed spontaneous bounty from the earth without labor. The festival aimed to recreate these lost conditions of innocence and universal plenty during King Saturn's reign.
Romans reformed Saturnalia in 217 BC following their crushing defeat by Carthage at the Battle of Lake Trasimene. They consulted the Sibylline Books and adopted Greek rites including public banquets and shouts of io Saturnalia instead of traditional more Romano customs.
Slaves received banquets usually reserved for masters and could wear pilleus caps marking freedmen status alongside freeborn citizens. Horace described December as liberty when slaves could disrespect masters without punishment threat while social hierarchy leveling remained temporary until the holiday ended.
Sigillaria occurred on the 19th of December according to Macrobius and Ausonius who called it sacra sigillorum rites. Pottery wax figurines made specially that day and candles gag gifts exchanged instead of valuable items marked social status contrary season spirit.
The statue of Saturn had its feet bound in wool throughout most of the year to symbolize restriction before liberation during the festival. This binding was removed specifically for Saturnalia as an act of freedom allowing priests to sacrifice with heads uncovered unlike normal Roman practice.