https://www.aish.com/atr/Sylvester_Day.html

Ask the Rabbi

I understand that in Israel the secular New Year is referred to as "Yom Sylvester." Where does this name come from?

In 46 BCE the Roman emperor Julius Caesar made adjustments to the Roman calendar, including beginning the new year on January 1 rather than in March. (He egocentrically decreed that the calendar should henceforth be called the "Julian" calendar.)

In practical terms, all cultures celebrate the new year according to their particular calendar and the Romans were no different. When the Roman Empire became the Holy Roman Empire under Constantine, at his mother Helena's behest, the Christian world carried on the custom of celebrating the Roman new year.

In many European countries this day was named after Saint Sylvester. There have been three popes named Sylvester (who later became Saints), but the one after whom the day is named is Sylvester I (314-335). Christianity grew under his rule and it is believed that he died on December 31. In addition, during his rule it was believed that he had been swallowed by the Leviathan sea monster and that the monster would return in the year 1000 to destroy and kill. When it did not, people were relieved and they celebrated.

As you see, there is nothing remotely Jewish about "Sylvester Day." So why is it celebrated in Israel?

Israeli society flows according to the Jewish calendar. Schools and businesses are closed on Shabbat, and the whole country shuts down on Jewish holidays like Yom Kippur. For that reason the secular/Christian new year has little significance. Yet when some ultra-secularists discovered that most of the world holds a "New Years party," they didn't want to feel left out.

Yet they couldn't call it "New Years" because that title was already taken by Rosh Hashana. So the name Sylvester was adopted in its stead.