For the one or two of you that do not know Hanukkah and Thanksgiving are the same day this year, you do now as we begin Hanukkah tonight with Thanksgiving tomorrow. Well-known speaker and professor Joel Hoffman sent out a commentary or narrative on this coalescing, and I have synopsized it for you.
The fourth Thursday in November can range from the 22nd to the 28th. On average, then, Thanksgiving falls on the 28th every seven years. 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 dates is that the Jewish calendar is 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 millennium C.E. ensures that the Jewish date and the secular date match up every 19 years.
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 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 to some extent this is true.
One hundred fifty two years ago (Abraham Lincoln was President) the first day of Hanukkah and the 4th Thursday in November were both on November 28th. But there was no Thanksgiving back then. 152 years from now, in 2165, Thanksgiving falls on the 28th, and you would expect Hanukkah also to fall on the 28th, but it does not, and this is where everything becomes complicated.
Remember, Shmuel fixed the details of the Jewish calendar. He thought the year was 365.25 days long. This is why we add a day every fourth February. But Shmuel was off by 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. But by the time of Pope Gregory XIII in 1582, those 11 minutes per year 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. Or, conversely, the spring equinox fell onMarch 11. Easter was shifting further and further away. 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. 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, did not care what Pope Gregory said. They kept using the Shmuelian calendar. 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 in the year 2165, when we would 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 do not 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!
Rabbi Jeffrey Abraham