Why is Rosh Hashanah, the Jewish New Year, so different from other Jewish holidays? On the face of it, it does not seem to follow any pattern. It is celebrated for two days, not only in the Diaspora but also in Israel. Yet the Sages refer to the two days of Rosh Hashanah as one long day – yoma arichta.
On Pesach, Shavuot and Sukkot we keep Yom Tov for two days because during the time of the Second Temple there was doubt whether the month preceding Yom Tov was a chodesh chaser of 29 days or a chodesh maleh of 30 days. But on Rosh Hashanah the doubt was exacerbated for the following reason: In the case of other festivals, such as Pesach, the emissaries the bet din dispatched to advise outlying districts of a chodesh chaser had 14 days to reach their destination. In the case of Rosh Hashanah, however, the emissaries had no time at all. In fact, as soon as the witnesses had testified on the 30th day of Elul that they had sighted the new moon, that very day was declared Rosh Hashanah. And on Rosh Hashanah the emissaries could not travel more than the techum Shabbat distance of two-thirds of a mile beyond Jerusalem. As a result, even people living inside Israel but outside of Jerusalem remained in doubt.
Even inside Jerusalem, confusion reigned. Nobody knew whether the witnesses who would testify to the sighting of the new moon would arrive on the day of the 30th, in which case Rosh Hashanah would be on the 30th day, or whether they would not arrive, in which case Rosh Hashanah would be on the 31st day. On the night immediately following the 29th day of Elul and on 30th day of Elul itself, people hedged their bets. They ceased work, went to the synagogue, recited the Rosh Hashanah prayers and blew the shofar, all in a tentative state of mind. Perhaps, they fretted, the witnesses will not come today, the 30th, and tomorrow, the 31st, will be Rosh Hashanah by default and a day’s work would have been wasted. But then again, perhaps the witnesses would come. So how could they risk working?
The Levites in the Temple fretted, too. If the witnesses would not arrive by Minchah time on the afternoon of the 30th, the Levites had to proceed to offer up the tamid, the afternoon sacrifice. But they did not know which Psalm to sing when doing so. Should they sing the special Rosh Hashanah Psalm, or the weekday Psalm? One year they chose the weekday Psalm only to see the witnesses arrive after Minchah and prove them wrong.
In this situation, the rabbis decided to dispel the doubt. They decreed that if witnesses would arrive after the afternoon sacrifice on the 30th day of Elul, their testimony would be ignored and the 31st day of Elul would be declared Rosh Hashanah. Furthermore, in order to provide certainty for the Levites and in order to prevent people from working on the 30th of Elul after Minchah time, the rabbis merged the 30th day of Elul with the 31st day, declaring them both one long day.
From this decree on, the two days of Rosh Hashanah, unlike the two days of Pesach, Shavuot and Sukkot, were no longer celebrated out of doubt, but out of certainty. This distinction between the status of the two days of Rosh Hashanah and the two days of other festivals has practical ramifications. For example, on Rosh Hashanah, one may not extend the techum Shabbat 4,000 amot in two directions, as one may on the two days of Pesach, Shavuot and Sukkot. Because the two days of Rosh Hashanah are merged into one yoma arichta, only one eruv techumim could be placed for both days to walk 4,000 amot in only one chosen direction. Similarly, the argument that an egg laid on the first day of Pesach, Shavuot or Sukkot could be eaten on the second day of these festivals, would not apply. An egg laid on the first day of Rosh Hashanah could not be eaten on the second.
Following the destruction of the Second Temple, the dilemma of the Levites was no longer a concern. Accordingly, Rabbi Yochanan Ben Zakkai decreed that the testimony of witnesses arriving after Minchah on the 30th of Elul would once again be accepted, thereby rendering Rosh Hashanah one day. If witnesses did not arrive by nightfall of the 30th, Rosh Hashanah would be two days. Rabbi Yochanan Ben Zakkai’s decree did not, however, apply to the Diaspora, where it could not be known on the 30th day, whether the witnesses had arrived or not. Accordingly, in the Diaspora Rosh Hashanah remained two days, by decree. The Babylonian rabbis who came to Israel applied the same decree to the land of Israel, even after the time of Rabbi Yochanan Ben Zakkai.