Why two days Yom Tov outside Israel and one day in Israel? The answer to this question depends on the answer to another basic question: How many days constitute a month in the Jewish calendar?
The rebirth of the moon, approximately every thirty days, determines the number of days in the Jewish month. The precise duration of the Jewish month is 29 days, 12 hours and 793/1080 parts of one hour. Because it is impractical to commence Rosh Chodesh in the middle of the day, the most pragmatic thing to do is to alternate the months of the year between 29 days and 30 days, respectively. That is how it is done today. A 30-day month is called “maleh” and is born with a two-day Rosh Chodesh. A twenty nine-day month is called “chaser” and is born with a one-day Rosh Chodesh.
Whether a month is chaser or maleh makes a crucial difference to our Jewish lives. If, for example, one mistakenly believed the month of Elul was maleh when in fact it was chaser, one might find oneself eating on Yom Kippur and fasting on a weekday. If one mistakenly believed the month of Adar was maleh when in fact it was chaser, one might find oneself eating bread on Pesach.
So who decides which month is 29 or 30 days? Today the issue is decided by the Jewish calendar, instituted in 385 CE by Hillel II. Prior to that, during the time of the Second Temple, a chodesh chaser was determined by the empirical evidence of two witnesses, who testified in the Jerusalem court of law, the bit din, that they had seen the new moon on the 30th day. Once their testimony was accepted, the thirtieth day, counted from the previous Rosh Chodesh, was declared the Rosh Chodesh of the new month. If no witnesses came on the 30th day, Rosh Chodesh was declared, by default, on the 31st day, rendering the previous month a chodesh maleh.
At first, the news of a chodesh chaser was spread on the night of the thirtieth by means of torching beacons on mountain peaks. If no torches were lit, all understood that the month was maleh. The Cutheans, however, disrupted this system by torching beacons on days when the Jerusalem bet din had not declared a chodesh chaser. In this situation, the only alternative was to dispatch emissaries to outlying districts in the hope they would arrive with the news of the chodesh chaser before Yom Tov. In the case of Rosh Chodesh Nissan, for example, the emissaries would have 14 days to arrive before Pesach. Those communities, too distant from Jerusalem for the emissaries to arrive before Yom Tov, such as communities in the Diaspora, were forced to hedge their bets and keep Yom Tov for two days. These two days are called Yom Tov Sheni Shel Galuyot. One of those days would genuinely be Yom Tov and one would not. Since at the time they did not know which one was which, they kept both.
That was then. But now we have Hillel’s calendar. There is no doubt. We know which day is Yom Tov. So why are we celebrating “Yom Tov” again on a “weekday”? The comforting yet chilling answer is to be found in Tractate Beitzah. “Hold on to the customs of your ancestors,” the Talmud advises, “for someday, an oppressive government may confiscate your Jewish calendar and forbid you to teach it. Then, having forgotten the methods of your ancestors, you will lose track of the days, and eat bread on Pesach.” And so, even in the darkness of Auschwitz and Soviet prisons, Jews celebrated Yom Tov based on the trustworthy tools of yesteryear.