A Consequence Of Our Exile
‘Guard The Custom Of Your Forefathers’
The Gaonim write that the practice of keeping two days of Yom Tov outside of Eretz Yisrael was already observed in the days of Yechezkel and Daniel, who kept two days of Yom Tov in Bavel (see Otzar HaGaonim, Teshuvos, on our sugya).
What is the origin of this custom?
In the time of the Beis HaMikdash, beis din determined the date of Rosh Chodesh based on the testimony of witnesses who observed the new moon. If no one testified, or if the sky was too cloudy for the moon to be seen, beis din determined Rosh Chodesh based on astronomical calculations (Rambam, Hilchos Kiddush HaChodesh, 18).
Sefer Ir HaKodesh V’HaMikdash 3:19 notes four different periods in history in regards to how Rosh Chodesh was determined and announced. The first period began with the construction of the second Beis HaMikdash and continued after its destruction. During this period, the Jews in Eretz Yisrael would light fires at night on mountain tops, thus signaling to Jews living in distant places that Rosh Chodesh was announced. The Jews who saw these signals would then light their own fires, signaling to their brethren in even more distant cities. Thus, the message would quickly spread throughout Eretz Yisrael and the Diaspora (see Mishnah, Rosh Hashanah 22b, cited in our sugya).
If for some reason a person in a far-off region did not hear the news, and was therefore uncertain when the next Yom Tov should be observed, he would observe the holiday on two consecutive days. He didn’t do so because of a rabbinic enactment; he simply was covering his bases due to his uncertainty.
Eventually the fire signals were abandoned because of unscrupulous people who signaled falsely, intentionally confusing the Jews in the Diaspora. Thus began the second period, in which Rabbi Yehuda HaNassi decided that mounted messengers would be sent on Rosh Chodesh as far as possible to announce when the next Yom Tov should be observed (see Yerushalmi, Rosh Hashanah 2:1).
People living in areas outside the messengers’ range were forced to constantly observe two days of Yom Tov out of doubt. Again, they weren’t following a rabbinic enactment; they simply were unsure what to do and kept two days to be safe.
The third period began in the second generation of Amoraim, in which Rabbi Elazar ben P’das cautioned Diaspora Jews to “guard the custom of their forefathers” and observe two days of Yom Tov. Even if they knew for certain when a certain Yom Tov should be observed, they should still celebrate the Yom Tov for two days due to a rabbinic enactment.
Enter The Modern Calendar
The fourth period began seven years after the death of Rava (4118) when Hillel, the last Nassi (not to be confused with Hillel the Elder, founder of Beis Hillel), established a calendar to be used by all future generations. By means of this calendar, Jewish communities throughout the world always know when a Yom Tov should properly be observed. Nevertheless, the rabbinic enactment of observing two days of Yom Tov in chutz la’aretz remains.
Why? The Gemara explains that gentile authorities may forbid us to study Torah and we may forget the mathematical basis of Hillel’s calendar. We may then start observing the Yamim Tovim on the wrong days and accidentally violate biblical prohibitions such as eating chametz on Pesach. Therefore, we must “guard the custom of our forefathers” and observe two days of Yom Tov.
However, this rabbinic enactment only applies to communities in chutz la’aretz that have always kept two days of Yom Tov. Those places in Eretz Yisrael within range of beis din’s messengers keep one day of Yom Tov as they always did. Our Sages did not wish to change the practice observed by each community before Hillel developed his calendar (Ran, Sukkah 44a s.v. Itmar; the Meiri adds that in Eretz Yisrael there was no fear that the Jewish people would be forced to cease studying Torah).