Prior to Gospel times, Saturnalia was one of the biggest holidays the pagans celebrated.  It began on Dec 17 and lasted for a "week".  A week is understood today almost universally to refer to a 7 day period, but not so before the days of Constantine (321 AD).  Romans had an 8 day week where they worked for 7 days and rested on the 8th.  The idea of a 7 day week was first introduced in 45 BC, but not actually used until a few centuries later. when Constantine killed the 8 day week for good.

In much of the Greek speaking world, Southeastern Europe, a 10 day week was used as well as in parts of the Mid-East durign the first few centuries BC and AD.  Egypt had a 10 day week in antiquity but changed to a 7 day week, and rested on the Jewish Sabbath by the time we get to the first century BC.

The poet Lucian of Samosata (120-180 AD) wrote in Greek, so most of his audience probably would have understood his reference to Saturnalia starting on Dec 17th and lasting for a "Week" to mean it lasted until Dec 26.  Romans might have unstood that to mean it lasted until Dec 24.  In short, it was a different length to different societies.

Several Roman Emperors tried to shorten the holiday to no success.  Augustus (d 14 AD) tried to limit it to 2 days.  Caligula (d 41 AD) tried to limit it to 5 days.  But they really only affected what government offices would be open when since these pagan societies loved their feast to the god of 6th planet and celebrated it for a week no matter what the government did, and apparently for whatever a "week" meant to their society.  But each city had its own calendar and own definition of a "week".

Somehow early Christians around Constantine's time (321 AD) chose a day near the end of this festival, at the end of the average length of a "week".  The 10 day Greek week stretches past this date, the 8 day Roman week ends the day before, but on average, it was at the end of the week.

Dec 17 was chosen in that it was the first day of the sign of Capricorn.

Shalom,

Joe