Home > current events, holidays, Judaism > Why Hanukkah and Thanksgiving will Never Again Coincide

Why Hanukkah and Thanksgiving will Never Again Coincide

[You can also read a version of this on the Huffington Post.]

Hanukkah-5774-Thanksgiving-2013Try to keep up with me on this.

I know that’s an ominous way to start, but it’s worth it.

This month, Hanukkah and Thanksgiving will overlap for a joint celebration that will never happen again. Here’s why.

Thanksgiving is the 4th Thursday in November. Hanukkah is the 25th day of the Jewish month of Kislev.

The 4th Thursday in November can range from the 22nd to the 28th. If the 29th is a Thursday, then so is the 1st, so the 29th would be the fifth Thursday, not the fourth. And if the 21st is a Thursday, then it’s only the third Thursday. On average, then, Thanksgiving falls on the 28th about every seven years. It will fall on the 28th this year, then again in 2019, 2024, 2030, and 2041, or four times in the next 28 years. (It’s not exactly every seven years because leap days throw things off a little.)

The Jewish month of Kislev can currently start as early as November 3 or as late as December 2, which means that the first day of Hanukkah can come as early as November 28 or as late as December 27.

The reason for the broad range of possible dates is that the Jewish calendar is lunar-solar. The months are based on the cycles of the moon. But the calendar changes the lengths of those months, and even how many months are in a year, to make sure that Passover always falls in the spring. This complex system — put in place by Rav Shmuel in the first half of the first millennium CE — ensures that the Jewish date and the secular date match up every 19 years. (By contrast, the Muslim calendar is purely lunar, which is why Ramadan can fall during any time of the solar year. The Christian religious calendar is almost entirely solar, but Easter falls on the first Sunday after the first full moon after the spring equinox [around March 21], a calculation that involves the moon as well as the sun.)

Because of this Jewish 19-year cycle, 19 years from now, in the year 2032, Hanukkah will again fall on November 28. But Thanksgiving in that year falls three days earlier, on the 25th.

On average, we would expect the 19-year Jewish cycle and the 7-year Thanksgiving-on-November-28 cycle to coincide about every 19×7 years, which is to say, approximately every 133 years. And they sort of do.

One-hundred and fifty-two years ago, in 1861, the first day of Hanukkah and the 4th Thursday in November were both on November 28th. But there was no Thanksgiving back then.

In 152 years from now, in 2165, Thanksgiving falls on the 28th, and you’d expect Hanukkah also to fall on the 28th, but it doesn’t.

If you you’ve been paying attention (and if you haven’t given up yet), you may have noticed that I said “currently” when I explained when Kislev can begin. Remember Shmuel, who fixed the details of our current Jewish calendar in the first place? He, like everyone else back then, though that the year was 365.25 days long. This is why we have a usual year of 365 days, but every 4th year we add a leap day in February to make 366.

But Shmuel — again, like everyone else — was off by a little more than 11 minutes. The year is not quite 365.25 days long, but, rather, closer to only 365.2425 days, or about 11 minutes shorter than 365.25 days. For a long time no one noticed those 11 minutes. For a longer time no one cared. But by the time of Pope Gregory XIII in 1582, those 11 minutes per year — or about 3 days per 400 years — had added up to about ten days.

This meant that March 21, which had once been the approximate date of the spring equinox, was now 10 days later than the spring equinox. Or, conversely, the spring equinox fell on March 11. This was a problem for the Church, because the springtime holiday of Easter was shifting further and further away from spring.

Pope Gregory fixed the problem in two ways. First, he lopped off 10 days from the calendar. For Catholics, the day after Thursday, October 4, 1582 was Friday, October 15, 1582. Secondly, he eliminated 3 leap days every four hundred years. He decreed that years divisible by 4 would still be leap years, unless they were also divisible by 100 but not by 400. So 1600 would be a leap year (divisible by 100 and by 400), but 1700 would not (divisible by 100 and not by 400). This became known as the Gregorian calendar, and it gradually spread through the Christian world.

In 1752, the British empire adopted the Gregorian calendar, making the day after Wednesday, September 2, 1752 not the 3rd but rather the 14th. (An 11th day was necessary because 1700 was not a leap year in the Gregorian calendar.)

The Jews, of course, didn’t give a damn what Pope Gregory said. They kept using the Shmuelian calendar for their calculations. The Shmuelian calendar and the Gregorian calendar have been diverging at the rate of about 11 minutes a year, or 3 days every 400 years. Furthermore, the year 2100 will be a leap year in the Shmuelian calendar (because it’s divisible by 4) but not in the Gregorian calendar (because it’s divisible by 100 but not 400). So not long after the year 2100, the Jewish calendar and the secular calendar will diverge by an additional 1 day — though the details are even a little more nuanced, because Shmuel used a simplification of the final Jewish calendar.

This is why (remember the question from several paragraphs ago?) in the year 2165, when we’d expect Thanksgiving and Hanukkah to coincide again, Hanukkah will actually be one day later.

And that is why Thanksgiving and Hanukkah will never again coincide.

Well, almost never. If the Jews don’t ever abandon the calculations based on the Shmuelian calendar, Hanukkah will keep getting later and later — moving through winter, then into spring, summer, and finally back into fall — so that tens of thousands of years from now they will again coincide. But long before then the springtime holiday of Passover will have moved deep into summer, so be on the lookout for a memo with a calendar update in the next several thousand years.

And in the meantime, don’t miss this opportunity to enjoy an exceedingly rare confluence of celebrations.

Happy Hanukkah. And Happy Thanksgiving.

  1. November 20, 2013 at 2:39 pm

    I knew there was a reason I didn’t become a mathematician. Thanks, as always, Joel for doing the hard work for which no sane person would ever volunteer.

  2. November 20, 2013 at 4:39 pm

    You’re welcome, Billy. And read carefully. There’s going to be a test on Wednesday night.

  3. garen218@aol.com
    November 20, 2013 at 5:53 pm

    Great information.. lost me on the math there for a awhile, but you summed it up beautifully.. Thanks

    • November 23, 2013 at 6:51 pm

      Glad you enjoyed it.

  4. Ben Zion Kogen
    November 20, 2013 at 6:38 pm

    Great piece — This is what mathematicians do???

    • November 23, 2013 at 6:53 pm

      Thanks. Before I decided to pursue linguistics and then focus on religion and the Bible, I came pretty close to becoming a mathematician. It’s nice to see those two endeavors overlap.

  5. Ronna Kay
    November 21, 2013 at 1:08 am


    • November 23, 2013 at 6:54 pm

      Sometimes it’s on that 14th read-through that the concepts finally “click.” :-)

  6. BZ
    November 21, 2013 at 8:34 am

    This is not an accurate explanation for the Chanukah drift. It conflates two different issues: the “Shmuelian” (equivalent to the Julian) calendar and the Metonic cycle.

    1) Yes, Shmuel set up a solar calendar that assumed the year was exactly 365.25 days. However, this is not the lunisolar Jewish calendar that we use today. Shmuel’s year is only used for two purposes: tal umatar (the prayer for rain that begins in the Diaspora on Dec 4 in this century, Dec 5 in the next century, etc.) and birkat hachamah (the prayer for the sun that is said every 28 years, on April 8 in this century, April 9 in the next century, etc.).

    2) To keep the Jewish holidays roughly in sync with the solar calendar, we use the Metonic cycle (named after the Greek astronomer Meton), which adds 7 leap months every 19 years. This relies on the fact that 235 lunar months (= 19*12 + 7) are approximately equal to 19 solar years.(*) Approximately, but not exactly: this represents a solar year of 365.2468 days. This is more accurate than the Julian/Shmuelian approximation of 365.25, but less accurate than the Gregorian 365.2425. THIS is the reason that the Jewish holidays (including Chanukah) slowly drift later. They drift later by about 1 day every 231 years (which is slower than the Julian/Shmuelian drift of 3 days every 400 years, or 1 day every 133 years).

    Happy Thanksgiving!

    (*) BTW, it’s an urban legend that “the Jewish date and the secular date match up every 19 years”. Sometimes they do, but due to other quirks of the Jewish calendar, e.g. mechanisms to make sure the holidays fall on certain days of the week, they can be off by up to 2 days.

    • November 21, 2013 at 10:07 am

      Thanks for that.

      You are right, of course, that what I’m calling the Shmuelian calendar wasn’t created in its entirety by Shmuel, any more than the Julian calendar was created in its entirety by Julius Caesar, and that I skipped the distinction between Shmuel’s year (used to calculate the date for the tal umatar prayer) and what it eventually became.

      Our pattern of 7 leap years every 19 is based on the Metonic Cycle, but (as I’m sure you know) we use our own variation, with six different kinds of year (three kinds of non-leap years, three kinds of leap years), to balance the dual goals of limiting calendar drift and avoiding undesirable combinations of Shabbat and holidays.

      And (again as it seems like you know), we have our own way of calculating the year, in part based on units of 1/18 minute (3 1/3 seconds) called a chelek.

      And so forth.

  7. BZ
    November 21, 2013 at 8:35 am

    Also, while Thanksgiving and Chanukah won’t completely overlap again (until the calendar loops all the way around), they’ll still partially overlap in 2070 and 2165, when Chanukah will begin at sundown on Thanksgiving Day. As a practical matter, for those of us who do Thanksgiving dinner at dinnertime, we’ll still have more opportunities to light Chanukah candles at Thanksgiving dinner.

    • November 21, 2013 at 10:29 am

      Yes. And speaking for myself, that will be more of a festive celebration than the one we have this year, except that I’m probably not speaking for myself, since it’s doubtful that I’ll live long enough to experience it.

  8. November 21, 2013 at 10:02 am

    I’m fairly sure it’s a typo [Because of this Jewish 19-year cycle, 19 years from now, in the year 2023], but in 19 years it will be 2032.

    • November 21, 2013 at 10:09 am

      And yet my spell check didn’t catch it…. :-)

      Thanks. I’ve fixed it.

  9. r. ziegler
    November 21, 2013 at 9:54 pm

    Whatever your holiday holyday may your god go with you

  10. November 22, 2013 at 1:09 pm

    Here’s another nice piece about Hanukkah and the calendars, from Ben Blatt at Slate.com: Happy Valentinukkah’s Day!.

  11. gsghert01
    November 24, 2013 at 11:40 am
  12. Kade Madison
    November 28, 2013 at 9:03 am

    The simpler answer is it won’t happen again for another 79k years

  13. December 6, 2013 at 5:34 am

    As I point out at http://sabbahillel.blogspot.com/2013/11/why-thanksgiving-will-not-be-on.html, the calendar will be changed within the next couple of centuries so that Passover will continue to be in the spring. As a result,Chanukah will be moved back to mid November for the earliest appearances. In fact, since Purim (a month before Passover) hit March 26 in 1910, we will be ready for “skipping” a leap year as soon as the Sanhedrin (Supreme Court) is reinstituted. That is because March 26 is after the vernal equinox and should be Purim. Thus the year would not have Adar II but would have Nisan with Passover on March 26. The following year would be a leap year.

    The last time this happened was 2005 and the next time will be 2024.

  14. December 6, 2013 at 8:51 am

    Actually as I point out on my blog, sabbahillel.blogspot.com, as of 1910, 15 Adar II was on March 26, after the equinox. If the Sanhedrin had been re-instituted, that may have been the year that the leap year would have been postponed to the following year. This last occurred in 2005 and will occur again in 2024. If leap year would be postponed, then since Passover would be March 26, the following year would have Passover before the equinox (March 16) which would not be allowed. Thus, that year would be a leap year and 15 Adar II would be on March 16 and Passover would be the following month on April 15. Similarly, Chanukah following the “skipped” leap year would be November 26 instead of December 26, while the Chanukah following the “new” leap year would be approximately 20 days later or December 6. This would reset the 19 year cycle and we would have to recalculate the following years.

    Of course for this to occur, the Mashiach would have come and we would no longer be in exile. As a result, Thanksgiving would no longer be immediately relevant.

  1. November 22, 2013 at 9:14 am
  2. November 26, 2013 at 2:17 pm
  3. December 1, 2013 at 1:45 pm

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