I understand that in ancient times the rabbis decreed that Jews in the Diaspora should celebrate holidays for two days, because of some confusion about the correct day to celebrate. Nowadays, however, we have a fixed calendar, so why do we still celebrate two days in the Diaspora?


Good question. Indeed, we could ask why the custom of celebrating two days didn’t stop almost 2,000 years ago, when the Jews started using a fixed calendar as opposed to determining the months based on observations of the new moon.

In Ancient Times

Before getting to the crux of the question, Let's first start with a brief overview of how Jewish holiday dates work:

Whereas Shabbat happens every seven days without any input from us, G‑d commanded the children of Israel to fix the dates of the months and years.1 This means that the power to determine when the holidays fall was given to Moses and his successors—the rabbinical courts.

Originally, there was no fixed calendar. Each month, the Sanhedrin (supreme court) in Jerusalem would determine whether the previous month had been 29 or 30 days long, depending on when the new moon of the following month was first sighted.

There was no way to determine the exact day of a coming festival (Yom Tov) in advance, because every festival falls on a particular day in a month, and the month would begin only when the new moon of that month was sighted.

(The way that the Sanhedrin determined the new month is fascinating in itself. Click here to find out more about it.)

Once the Sanhedrin had determined that a new month had begun, the information was broadcast from Jerusalem to distant Jewish outposts via huge bonfires which were lit on designated mountaintops. Lookouts stationed on other mountaintops would see the fires, and would then light their own fires, creating a chain of communication that led all the way to Babylon, and to even more distant communities. If there was a Yom Tov that month, communities across Israel and in the Diaspora would then know when to celebrate it.

But a problem arose. The Samaritans, a sect who denied rabbinic authority and were constantly at odds with the Jews, started lighting fires on the wrong days in order to manipulate the calendar.

To prevent confusion, the fire-on-mountaintop method of communication was discontinued, and instead messengers were dispatched to Babylon and other farflung Jewish settlements.

Since news traveled a lot slower that way, distant communities would not know when Rosh Chodesh (the “Head of the Month”) had been declared in time to celebrate the festival on the proper day.

It was therefore decreed that outside of the Land of Israel people would celebrate every Yom Tov for two days: the day of the month the holiday would be if the previous month had been a 29-day month, and the day of the month it would be if the previous month had been a 30-day month.

The Custom of Your Forefathers

The fixed calendar began being used in the 4th century CE, and so everyone knew in advance when Rosh Chodesh and the festivals would occur. However, the Talmud explains that although the doubt about the calendar is no longer relevant, we are still bound according to rabbinic law to observe a second day, in case a doubtful situation were to arise again. In the words of the Talmud, “The sages sent [word] to the exiles, ‘Be careful to keep the customs of your forefathers, and keep two days of the festival, for someday the government may promulgate a decree, and you will come to err.’”2

However, the question remains. A simple reading of the Talmudic text cited above seems to indicate that the reason to still keep two days is merely as a precaution in case we are not able to learn the Torah and the proper way to calculate the new month. However, nowadays, with the advent of modern technology and computers, we have calendars stretching for hundreds of years into the future, so this would hardly seem to be a concern.

An Enactment of the Prophets

Rabbi Hai Gaon was the undisputed authority on Jewish law in the early 11th century. He writes that the requirement that one should keep a second day of Yom Tov outside of Israel really stems from the days of the prophets, and perhaps even from the days of Yehoshua (Joshua) ben Nun.

Based on this, he explains the somewhat curious wording of the Talmud cited earlier: “Be careful to keep the customs of your forefathers and keep two days of Yom Tov.” Why not state simply that we should keep two days lest there develop some doubt as to the correct day?

However, there is a general rule that once a rabbinical enactment has been made by the Sanhedrin and accepted by the entire Jewish people, the enactment can be absolved only by a court that is similar or greater in number and stature to the one that made the enactment. While one would be hard-pressed to find a court that could compare to even a regular Sanhedrin, it would be impossible to find a court with the same stature as the prophets, who were divinely inspired. Thus the Talmud warns that we should “be careful to keep the customs of your forefathers,” as the custom of keeping two days of Yom Tov in the Diaspora isn’t just about satisfying a doubt, but about adhering to an enactment instituted by the prophets.

Rabbi Hai Gaon concludes that like many of the enactments of the prophets, we often do not know the real reason or “secret” behind their enactment.3

The Holiness of the Land of Israel

According to the teachings of the inner wisdom of Torah, there is a deeper reason—perhaps even the “secret” of the prophets to which Rabbi Hai Gaon alludes—for keeping two days of Yom Tov in the Diaspora.

The Tzemach Tzedek, the third Lubavitcher rebbe, expounding on what Kabbalists such as Rabbi Moshe Cordovero and others write, explains that since holiness is more revealed in the Land of Israel, the festivals can be revealed and received there in one day, as the Torah commands. However, those in the Diaspora are farther away from the revelation of holiness, and therefore, in order to absorb the spiritual emanations of the festivals, two days are required, as the rabbis ordained.

The Tzemach Tzedek explains this phenomenon using the metaphor of a bright torch: When held close to an object, the light is strong and concentrated on a small area; but when the torch illuminates an object from a distance, its light is weakened and dispersed over a large area. Thus, the light of the festivals is revealed in the Land of Israel in one concentrated and focused day, while in the Diaspora the light of the festivals is weaker and spread out over two days.4

Perhaps it is for this reason5 that the Lubavitcher Rebbe,6 as well as others,7 contends that even in the days of Moshiach, when the Holy Temple is rebuilt and the Sanhedrin is reestablished, those outside of Israel proper will continue to celebrate Yom Tov for two days. After all, although the whole world will be elevated, the Land of Israel will be elevated with an even greater degree of revelation. May this be speedily in our days!

Please note: Regarding the law for travelers to or from Israel, although some authorities say that you should keep Yom Tov for one day if you are in Israel and two days if you are elsewhere, most authorities rule that wherever you are, you keep Yom Tov for the number of days it is kept in the country of your permanent residence. One should consult with one’s own rabbi before traveling.