What are these pagan holidays?
Rav Ḥanin bar Rava explains that Kalenda refers to the holiday that is celebrated for eight days following the winter solstice, while Saturnalia is the eight day festival that precedes it.
Kalenda or Calenda usually refers to the first day of the month according to the Roman calendar, but in our case the Sages are talking about the first day of the first month of the year – Kalendae Januirae – that is to say, the first day of the month of January. As the Gemara explains, the celebration of this festival began immediately following the winter solstice on December 22 and lasted for eight days. As part of the celebrations the Roman would bring sacrifices to the pagan gods and arrange for games and related activities at the circus.
Saturnalia became one of the most popular Roman festivals. It was marked by sacrifices to the god Saturn and general revelry that included reversal of social roles, in which slaves and masters ostensibly switched places.
Originally celebrated for a day, on December 17, its popularity saw it grow until it became a week long extravaganza, ending on the 25th day of the month.
Saturnalia involved the conventional sacrifices, a couch (lectisternium) set out in front of the temple of Saturn and the untying of the ropes that bound the statue of Saturn during the rest of the year. A Saturnalicius princeps was elected master of ceremonies for the proceedings. Besides the public rites there were a series of holidays and customs celebrated privately. The celebrations included a school holiday, the making and giving of small presents (saturnalia et sigillaricia) and a special market (sigillaria).