Extra Festival Days in the Diaspora
Israelis and liberal Jews observe fewer days for some holidays than traditional Diaspora Jews.
In the Torah, major Jewish holidays are shorter than what most traditional Jews outside Israel celebrate now. So we read, "Seven days you shall eat unleavened bread" (Exodus 12:15) and celebrate eight days of Passover. The festivals of Shavuot, Yom Kippur, Rosh Hashanah, and Shemini Atzeret are listed as single days in the Torah.
How did Diaspora practice diverge from the Torah's instructions? The answer lies in our history, during the time of transition from biblical to rabbinic Judaism around the beginning of the common era.
The Calendar Needed Witnesses, Beacons, and Messengers
The Jewish calendar is lunar. Over 2,000 years ago, a council of rabbis from the Sanhedrin, the ancient legislative and judicial body, held special sessions in Jerusalem at the end of each lunar month to receive witnesses to the first sliver of the new moon. Because a lunar cycle is approximately 29 days long, it was no mystery when the new moon should appear, but the Sanhedrin still declared months and holidays only on the basis of these witnesses. To encourage ordinary people to take the time to come and testify, they were fed and honored. The rabbis questioned every witness for credibility:
'Where did you see [the new moon]? Was it before or after sunset? Was it in the north or the south? How high was it? How high was it? Which way was it tilted?" (Mishnah Rosh Hashanah 2:6).
Once the sighting was legitimated, the rabbis declared the next day Rosh Hodesh, the beginning of the new month. Originally, beacon fires would be set on mountaintops to spread the word to distant Jewish communities already living in far away places such as Egypt and Babylon. Watchers on faraway hills set their beacon fires as soon as they saw them, continuing the relay "until one could behold the whole of the Diaspora before him like a mass of fire" (Mishnah Rosh Hashanah 2:4).
But relations with neighboring sects such as the Samaritans worsened, and they deliberately harassed the Jews by lighting beacon fires at erroneous times. As a result, the Sanhedrin substituted messengers to alert the Diaspora communities, but they could take a long time to arrive from Jerusalem.
Only the Sanhedrin was able to make pronouncements about the new moon, so Diaspora communities could not do this for themselves. The proclamation of each new moon was significant as an alert about the exact dates of holidays. Jewish communities always knew approximately when a festival would fall, but waited until the date of the new moon had been established to decide when festivals would fall. As most holidays fall around mid-month, they could--generally--afford some delay in receiving the Jerusalem news. But celebrating festivals for an extra day would ensure that, regardless of whatever confusion reigned about the exact start of the new month, at least one day of their celebration would be on the correct day.