How often does the first night of Hanukkah fall on Christmas?

This year, the first night of Hanukkah happens to fall on Christmas Eve — which hasn’t happened in nearly four decades.

On the Jewish calendar, which relies on lunar months that last either 29 or 30 days, Hanukkah begins on the same night every year: the 25th of the month of Kislev. But the Jewish calendar doesn’t quite line up with the secular, solar-based Gregorian calendar. As a result of a differentiation in calendar systems, Hanukkah’s start date in the United States fluctuates wildly year to year, falling anywhere between November 27 and December 26 on the Gregorian calendar.

I was curious: Just how common is it for the first night of the Jewish holiday to occur on Christmas Eve, like this year, or Christmas Day?

Using online calendars chronicling the historical dates of Jewish holidays, I compiled more than 100 years of Hanukkah data. Since 1900, the first night of Hanukkah has fallen on Christmas Eve three times (with 2016 being the fourth). Hanukkah has started on Christmas Day four times in the same period.

With eight total occurrences in 117 years, Hanukkah starts on Christmas Eve or Christmas Day roughly once every 15 years. Not so rare!

Looking at the most common start dates of Hanukkah since 1900, you can see that Hanukkah typically starts the week before Christmas.

With six total occurrences, though, December 7 has historically been the most common start date of Hanukkah. But since 1900, Hanukkah has typically begun in the 10 days preceding Christmas Day.

Of course, as I mentioned, this is all a result of calendrical differences. Despite the fluctuation we see on the Gregorian calendar, Hanukkah always begins on the 25th night of the month of Kislev on the Hebrew calendar.