Question: I am trying to determine the correct feast dates for 1999. I am very confused, because I have several dates that vary greatly. There is one group that dates them about one month after the ones given at your site! Another site dates the fall feasts the same as yours, but their spring feast are one day later (March 31, April 2-8, etc). Can you please tell me how you reckon your dates?
Reply: The Bible is vague about the rules for calculating the calendar. The Bible presents only general principles. This must be the case as soon as we think about the practical realities of maintaining a calendar in the scientifically-primitive agricultural society of ancient Israel.
For much more than a thousand of years, the religious calendar of Israel/Judah was maintained by observation of the moon, with suitable adjustments (postponements, i.e., addition of extra days to months, and intercalations, addition of an extra month in the year) brought on by practical necessities, such as the availability of sufficient lambs for the Passover. This calendar was self-correcting, i.e., mistakes could not accumulate, but would automatically be corrected either next month or next year.
Gradually society became more sophisticated (civilized) and computation was introduced as a cross-check in case of bad weather etc. Persecution of the Jews under the Roman Emperor Constantius (337-361) in the name of Christianity prevented communication among the Jews about the observed calendar, and forced them to switch to an entirely computed calendar. This forced Hillel II to promulgate the fixed computed calendar in 358. Its computations were designed to simulate the practical constraints of the observed calendar (including postponements and intercalations) as closely as possible. This computed calendar has been used by most Jews unchanged for the last 1,000 years. Except in the matter of the date of Pentecost, this is the calendar ABCOG follows (see http://www.abcog.org/shavuot.htm for details of Pentecost).
The Karaite Jews still try to go by observation.
Recently, the use of computers with astronomical information, has meant that more astronomically correct calendars can be computed. However, these depend on defining rules, such as "When is sunset?", "When is a New Moon visible?" Different rules lead to different calendars. There is no consensus as to which computer program, astronomical information, and set of rules is correct. In other words, all computed calendars are speculative.
What if the calendar is not managed correctly? Surely this happened sometime during the Old Testament. Yet the prophets never castigate the priests for calendar errors. Only Jeroboam is censured for deliberately changing the Feast of Tabernacles from the 7th to the 8th month. In the Talmud, (tractate Rosh HaShanna), the Rabbis debate what would be God's action if the calendar was computed incorrectly. They conclude that (i) if precise dating was supremely important, God would have to control the calendar much more strictly, (ii) God is big enough to handle errors in computation.
To this time, there is no overwhelming scientific or religious motivation to depart from the well-established, time-tested Jewish computation. (see http://www.abcog.org/saadia.htm for an example of an argument about calendar computation).

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