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The additional constraints are religious in origin and are expressed in the form of four rules of postponement, or dehiyyoth, which represent both certain constraints and the logical consequences of the constraints. |
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The rules of postponement are: |
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1.) When the Molad Tishri occurs on a Sunday, Wednesday, or Friday, Rosh Hashanah is postponed to the following day. |
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2.) When the Molad Tishri occurs at hour 18 (noon) or later, Rosh Hashanah is postponed until the next day. Dehiyyah 1 is reapplied to the result of this rule. |
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3.) When the Molad Tishri of a common year falls on Tuesday, 204 halakim after the ninth hour, Rosh Hashanah is postponed until Wednesday. Dehiyyah 1 is reapplied to the result of this rule, resulting in a further postponement to Thursday. |
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4.) When, in a common year succeeding a leap year, the Molad Tishri occurs on Monday morning, 589 halakim past the 15th hour or later, Rosh Hashanah is postponed to the next day. |
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These rules appear to be both arbitrary and specific. The actual situation that causes the rules is somewhat simpler. The intention of Dehiyyah 1 is to implement three religious requirements: |
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Yom Kippur (Tishri 10) shall not fall on the day before the Sabbath. |
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Yom Kippur shall not fall on the Sabbath. |
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Hoshana Rabba (Tishri 21) shall not fall on the Sabbath. |
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Dehiyyah 2 is a codification of the rule of thumb that governs the relationship between the new moon and the first visibility of the lunar crescent. |
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The other two dehiyyoth, 3 and 4, are mathematical consequences of the application of the calculated lunation length and the rules of intercalation to the requirements expressed by 1 and 2. Rules 3 and 4 are not, therefore, necessary from one standpoint, since they may be deduced logically. They are, in effect, rules of thumb which eliminate the need to work certain cases through logic and guarantee that the rules have been applied. |
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Note that the dehiyyoth are presented as exceptions to the rule. In more than 60 percent of all years, however, Rosh Hashanah does not occur on the day of the Molad Tishri but is postponed by one or more of the dehiyyoth. In practice, postponement is the rule, rather than the exception. |
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Another further consequence of the rules presented to this point is the fact that all of the years in the Jewish calendar are constrained to fall into one of 14 categories or year types, called kebioth. In the Hebrew literature, the kebioth are coded by three letters in the following way: |
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