Why is Rosh Hashanah 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 yoma arichta – one long day.
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 as Rosh Hashanah. And on Rosh Hashanah the emissaries could not travel more than the techum Shabbat distance beyond Jerusalem. As a result, even those living inside Israel but outside of Jerusalem remained in doubt.
And confusion reigned even inside Jerusalem. Nobody knew whether the witnesses who would testify to the sighting of the new moon would arrive on the day of the thirtieth, in which case Rosh Hashanah would be on the thirtieth day, or whether they would not arrive, in which case Rosh Hashanah would be on the thirty-first 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 would not come that day, the thirtieth, so the following day, the thirty-first, would be Rosh Hashanah by default and a day’s work would have been wasted. Then again, perhaps the witnesses would come that day. So how could they risk working?
The Levites in the Temple also fretted. If the witnesses would not arrive by Minchah time on the day of the 30th, the Levites had to proceed to offer up the tamid, the afternoon daily 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 bet on 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 of Elul would be declared Rosh Hashanah. Furthermore, in order to provide certainty for the Levites and 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, or yoma arichta.
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 the other festivals has practical ramifications. For example, on Rosh Hashanah one may not extend the techum Shabbat 4,0000 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 techumin could be placed for both days to allow for walking 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 Rosh Hashanah could not be eaten on the second.