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The Jewish calendar is based on a lunar system in which each month begins at the new moon. A month can consist of 29 or 30 days.
Originally the determination of the new moon was by the Sanhedrin (highest rabbinic court, 70 members) when witnesses declared that they had seen the new moon. Once the Sanhedrin declared the new month, messages were sent to the various communities stating which day (of the two possible) began the new month.
Note that this does not say that the Sanhedrin did not know the methods of calculating the new moon, but merely that proper procedure required the witnesses. The astronomical calculations could be used to verify the validity of the witnesses, if necessary.
In those months in which holidays occurred, (such as Nisan for Pesach), the exact date was critical for determining when the holiday began. Since the messengers could not reach every community in the time allotted, those communities they could not reach would celebrate both possible days.
In 325 CE, the Byzantine (Eastern Roman Empire) administration in what was then Palestine limited the rights of the Sanhedrin to disseminate calendrical information. Hillel II, the Sanhedrin President at the time, published a set of rules for the maintenance of the calendar, which is still used today.
The Sanhedrin also determined that those communities outside of Eretz Yisrael would continue to keep the second day of the holiday because of a principal "Minhag Avoseinu Beyadeinu", the customs of our fathers [remain] in our hands, and to remember the law for when the Sanhedrin is reestablished and the month is determined by witnesses again.
Reform Jews believe that since the calendar is defined exactly, there is no longer a need to keep the second day of Yom Tov outside of Eretz Yisrael. However some Reform Jews do choose to observe two days.
Many Orthodox Jews feel that once the Sanhedrin is reestablished, that the declaration will be disseminated by CNN [Cable News Network] or maybe even soc.culture.jewish, and even the Jews outside of Eretz Yisrael will only be required to keep one day. However, until that time comes, the requirement is to maintain the calendar as established by the original Sanhedrin, which includes two days.
The FAQ is a collection of documents that is an attempt to answer questions that are continually asked on the soc.culture.jewish family of newsgroups. It was written by cooperating laypeople from the various Judaic movements. You should not make any assumption as to accuracy and/or authoritativeness of the answers provided herein. In all cases, it is always best to consult a competent authority--your local rabbi is a good place to start.
Hopefully, the FAQ will provide the answer to your questions. If it doesn't, please drop Email to firstname.lastname@example.org. The FAQ maintainer will endeavor to direct your query to an appropriate individual that can answer it. If you would like to be part of the group to which the maintainer directs questions, please drop a note to the FAQ maintainer at email@example.com.
© (c) 1993-2002
Daniel P. Faigin <firstname.lastname@example.org>