Why is Thanksgiving 2013 different from all others?

Because this year, it's not just Thanksgiving. It's also Thanksgivukkah.

For the first time since 1888—and the last time until the year 79,811—the first day of the Jewish holiday of Hanukkah will coincide with the American observance of Thanksgiving. By contrast, Hanukkah—which commemorates the re-dedication of the Temple in Jerusalem after the Maccabean military victory over the Syrians in 168 B.C.—far more typically occurs closer to Christmas, in December.

Thanksgivukkah, as this year's double-holiday convergence has been dubbed, is happening because of the difference between two calendars.

The date of Thanksgiving is determined by the Gregorian calendar, which is solar, while Hanukkah is set by the Hebrew calendar, which is lunisolar. In the lunisolar calendar, months are calculated according to the moon and the years according to the sun, with leap months added every few years to keep the seasons in sync. The different calendars explain why Jewish holidays—which occur every year on the same day in the Hebrew calendar—fall on different days of the Gregorian, or Western, calendar followed in America.

But there's another wrinkle. The Hebrew calendar is slipping "ever so slowly" behind the solar calendar, at the rate of about four days every thousand years, says physicist Jonathan Mizrahi, of Sandia National Laboratories in Albuquerque, New Mexico. Although the Hebrew calendar system, which was set over a thousand years ago, is a "very good calendar," says Mizrahi, "there are imperfections."

That slippage means that 200 years from now, Hanukkah won't ever begin earlier than November 29. Because November 28 is the latest day that Thanksgiving can fall, the two holidays won't overlap again until the year 79,811.

But according to Mizrahi, the far-off date is tongue-in-cheek because Jewish law requires that the holidays fall during specific seasons. The 79,811 date is predicated on the assumption that no adjustments will be made in the Hebrew calendar. "And that can't happen because once the holidays shift sufficiently far from the seasons in which they should occur, the calendar will have to be modified," he says.

Still, this year's Thanksgivukkah is a once-in-a-lifetime holiday. Indeed, the infrequency of the double-holiday convergence makes such rare astronomical phenomena as the transit of Venus—which last occurred in 2012 and won't reappear until 2117—almost common, by comparison.

Connection Beyond Coincidence

Even beyond coincidence, the convergence serves as a reminder of the "profound connection" between the origins and meaning of both holidays, says David Kraemer, professor of Talmud and rabbinics at the Jewish Theological Seminary in New York. The 17th-century Pilgrims "quite clearly modeled their celebration" of thanks for crops that would sustain them through the winter on the biblical fall harvest holiday known as the Festival of the Tabernacles, or Sukkot, he says.

Moreover, Hanukkah itself was a belated celebration of Sukkot. Unable to observe Sukkot at the proper time because they were in the midst of fighting, the Maccabees celebrated instead at the time of the rededication of the Temple, which is Hanukkah. Hanukkah begins on 25 Kislev, which is a bit more than two months after Sukkot, which begins on 15 Tishrei. "So their coming together is totally appropriate," Kraemer concludes.

Also appropriate is a certain overlap in menu traditions—think potatoes (whether mashed, dotted with marshmallows, or grated and shaped into pancakes). The potato commonality has to do with which local foods are available and in season at the time of the fall harvest in New England, where Thanksgiving originated, and in Eastern Europe, where the ancestors of the majority of American Jews came from.

Avoiding the "December Dilemma"

Is there an unexpected benefit to Thanksgivukkah? Because of their common theme of gratitude, pairing Hanukkah with Thanksgiving, rather than Christmas, calls forth no equivalent to the so-called "December Dilemma," when interfaith families strive to honor differing religious traditions and beliefs.

On the contrary. Back in 1888—the only other time Hanukkah and Thanksgiving converged—the weekly Jewish American newspaper, The American Hebrew, "encouraged readers to enthusiastically embrace both holidays, because Hanukkah is itself a holiday of Thanksgiving," says Dianne Ashton, author of the recently published book, Hanukkah in America: A History. The message was, "when it's Thanksgiving, you can completely join in with American society" to celebrate this ecumenical holiday, she says. "But not with Christmas, because it is a Christian holiday."

Ultimately, "Thanksgiving is a civic holiday with spiritual overtones," says Rabbi Julie Schonfeld, executive vice president of the Rabbinical Assembly, the international association of Conservative rabbis. Regardless of your religion or lack of faith, the holiday "speaks to gratitude, to forces greater than ourselves, and calls upon us to help others," she points out.