Chapter 17 

Technical Notes 

The Modern Jewish Calendar 

    The months in the Jewish calendar follow the cycles of the moon. Each month begins with the new moon. 

The length of each month is either 29 or 30 days, because the length of each lunar cycle (from new moon to 

new moon) is between 29 and 30 days. Lunar cycles do not have one set length; they vary in length. The usual 

number cited for the length of the lunar cycle is 29.5306 days. However, that number is an average of the 

length of the lunar cycle over a number of years. The actual length of the lunar cycle goes through a cycle of its 

own. The lunar month increases in length as each month passes, then the lunar month decreases in length, and 

then the cycle repeats itself. Any particular lunar cycle will rarely  have an exact length of 29.5306 days; but 

rather, the length of the lunar cycle moves through sets of months that are longer or shorter than that average 

length. 

    The length of 12 lunar months is approximately 354.37 days, which is approximately 11 days less than the 

length of the solar year, 365.25 days. In a purely lunar calendar, the months would recede though the seasons. 

Each month would begin about 11 days earlier each year, compared to the solar year. The months would 

move in relation to the seasons, so that a month which occurred in spring would occur earlier each year, 

eventually moving from spring to winter to fall to summer and back to spring again. 

    The Jewish calendar has 12 months of 29 or 30 days, leading to a year which is about 11 days shorter than 

the solar year. As a result, every 2 or 3 years, the Jewish calendar must add a 13th month, otherwise the feast 

of Passover would gradually move from spring into winter, and the other feasts and holy days would recede 

through the seasons also. 

    To prevent this from occurring, the Jewish calendar is periodically adjusted to stay in synch with the solar 

year. Every 2 or 3 years, a 13th month is added to the calendar. The month of Adar, which usually occurs just 

before the month of Nisan (when Passover occurs, in the spring) is repeated as Second Adar (AdarII). In the 

modern Jewish calendar, 7 of every 19 years are leap years. Thus 12 of every 19 years are about 11 days short 

of a solar year, but 7 of every 19 years have an additional 30 days added to the calendar. In those leap years, 

AdarI has 30 days (instead of the usual 29) and AdarII has 29 days, causing 30 days total to be added to the 

calendar in a leap year. Thus leap years are about 19 days longer than the solar year (instead of 11 days 

shorter). Those 7 leap years add enough days to the calendar, so that every 19 years has an average calendar 

year which is about the same length as the solar year. The cycle of 19 years, used to determine leap years, is 

called the Metonic Cycle. 

    The  exact length of each common year and each leap year, in the modern Jewish calendar, varies. A 

common year can actually be 353, 354, or 355 days in length (12, 11, or 10 days shorter than a solar year) and 

a leap year can actually be 383, 384, or 385 days in length (18, 19, or 20 days longer than a solar year). The 

reason for the variation in a year's length is that certain months can be either 29 or 30 days (such as Heshvan 

and Kislev), so that adding a leap year is not the only way to adjust the length of the year. The end result is 

that the Jewish calendar stays more or less in synch with the solar year and the Passover remains in the 

springtime (when grain is harvested in Israel).

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The Ancient Jewish Calendar 

    In ancient times, the Jewish calendar was adjusted periodically by the addition of the leap month of AdarII 

every 2 or 3 years. However, the leap years did not follow the Metonic Cycle. The decision as to which years 

would be leap years was a human decision, so that leap months were added as needed to keep the Passover 

celebration from receding from spring to winter.

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    Similarly, in ancient times, the Jewish calendar did not have a set length for each month. Any month, 

including leap months, could be 29 days or 30 days, as needed to keep the months in synch with the lunar 

cycle.

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 The length of each month, 29 or 30 days, was also determined in part by the desire to keep certain 

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