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The Jewish Calendar

By Larry Fine

The Jewish calendar is the perfect solution to telling time. However it is a bit more complex than the common calendar in use today in the Western world.

When ancient man realized the need for counting time he saw several options. The cycle of the sun was the most obvious; the sun rises and sets each day, therefore the day was defined by the sun. Yet how was he to mark a longer period?

How and why almost all mankind have accepted the seven day weekly cycle has no parallel structure in nature that would cause man to select the “week” as being a unit of time. Why man did not select the five day unit for a “week” or a ten day unit to be a “week” can only be explained through the influence of the Hebrew Bible where it is mentioned that in six days G-d created the world and on the seventh day he rested. This became the source for the seven day “week”.

The Month and the Year

Man’s life cycle was based on the seasons and the sun. He needed another frame of measuring time which he calls the “month” and a longer one which he calls the “year”. The year was necessary for the tracking of time and the working of the soil. The month was important for his festivals.

Today there are various cultures that use the solar cycle to measure the year and then sub-divide the year into 12 almost equal units and these are the months. Obviously this is the current method of measuring time in the Western world. The lunar cycle is completely ignored.

The sun moves up and down in the south sky and the time of the year changes. It takes 365.24 days for it to make such a cycle; hence the year is generally 365 days (except for leap years). A month is one twelfth of that, or 30.4 days, hence to even it out there are 7 months of 31 days, 4 months of 30 days and one, February, which are either 28 or 29 days.

Other civilizations utilize the moon as the chief mode of telling the time. Hence a lunar cycle is 29.5 days; therefore the month is either 29 or 30 days. As is well known, the Muslim calendar calls a year twelve months. Therefore they have a year of 354 days; eleven days short of the solar year. This causes their holidays to slip back each solar year. The solar cycle is ignored in that the month is not related to the yarly solar cycle.

The Jewish Method

How do the Jews deal with the problem of integrating the sun and moon?

First it must be stated that the Hebrew calendar is not an invention of the Jews. Since G-d told them that they must celebrate Passover in the spring, they can not have a sliding calendar like the Muslims. Since the requirement for Passover is that it falls on the fifteenth of the lunar month, they must incorporate also the lunar calendar.

The Hebrew calendar incorporates both the cycle of the sun and the moon into one useable calendar. The solar cycle retains the importance in regard to the measuring of the agricultural times and the lunar cycle is used for the marking of the festivals.

The Jewish calendar is dependent on the moon. Each month begins with the new moon. During the times when the Jews lived in the land of Israel, the new months used to be determined by observation. When people observed the new moon, they would notify the special court called the Sanhedrin. When the Sanhedrin heard testimony from two eyewitnesses that the new moon had occurred, they would declare the new month had started.

The current Jewish calendar is generally said to have been set down by the head of the Sanhedrin Hillel II in approximately 350 C.E. It is a remarkable and ingenious piece of calculation that was made before the calculator. Once the calendar was codified, the visual testimony is no longer accepted as the means of knowing the time of the new moon. It is accepted by Jews of all persuasions and is part of the official calendar of the State of Israel.

The Possibilities of the Jewish Year

There are nominally twelve months in the Hebrew calendar. On the occasion of a “leap year” the last month of the year “Adar” is repeated. When this happens the first Adar is called “Adar I” and the second is called “Adar II”. In the Jewish calendar, the leap year adds a 30 day month to the year.

The Jewish year is never uniform. It can vary from 353, 354, 355, 383, 384, or 385 days as shown in this chart:

Three Types of Years

The types of years are referred to as follows: the six single year lengths give rise to the terms chaser, kesidrah, and shalem.

Chaser means that the years are either 353 or 383 days long. In this type of year, the Kislev will have only 29 days. The term chaser means lacking or deficient. The Hebrew letter chet is used to denote these year lengths.

Kesidrah is the term applied to Hebrew years that are is either 354 or 384 days long. Such year lengths are accommodated by keeping all of their month's lengths intact. The term kesidrah means regular or normal. The Hebrew letter chof denotes one of these year lengths.

Shalem is the term applied to Hebrew years that are is either 355 or 385 days long. This is accomplished by adding a 30th day to the month of Heshvan. Shalem means complete. The Hebrew letter shin is used to denote these years.

* Note: For leap years the 30-day month of Adar I is added immediately after Shevat and followed by Adar II with 29 days. In non-leap years, Adar has only 29 days.  

  Notes:
  * In a complete year, Heshvan has 30 days.
  ** In a deficient year, Kislev has 29 days.
*** In a leap year Adar I has 30 days; it is followed by Adar II with 29 days.

The Nineteen Year Cycle

The Jewish calendar incorporates a cycle of 19 years. This consists of 12 years of 12 lunar months each and 7 years of 13 lunar months each for a total of 235 lunar months. The Hebrew name for this cycle is “Mahzor Katan”, the small cycle. Years 3, 6, 8, 11, 14, 17, and 19 of the 19 year cycle are leap years of 13 months each. The reason for this was to insure that all Hebrew years in the 19 year cycle would begin less than one lunar month after the start of their corresponding solar year in that cycle.

The 19 year cycle does mean that the Hebrew calendar repeats itself every 19 years. The 19 year cycle only refers to the positions of the 13-month years in those cycles as mentioned above. All periods of 19 year cycle can be either 6938, 6939, 6940, 6941, or 6942 days each. No two consecutive periods of 19 years can begin on the same day of the week since none of these values are exact multiples of 7.

The distribution is easily remembered by the Hebrew mnemonic device: GUCh ADZaT which stands for the Hebrew letters gimel-vov-het aleph-daled-zayyen-tet.

Another rule to know is that a (13-month) leap year cannot follow another leap year.
Neither can regular years cannot follow regular years nor can deficient years cannot follow deficient years. However 384-day years are always followed by 355-day years.

Distribution of the Months

Although the year begins with Tishrei, the months begin with Nisan. The Hebrew months basically alternate between 30 and 29 days as follows:

For leap years the 30-day month of Adar is added immediately after Shevat. In a leap year Adar I has 30 days; it is followed by Adar II with 29 days.  In non-leap years, Adar has 29 days.  Before the calendar was fixed the months could either be 29 or 30 days. It depended upon the sighting of witnesses. Once the calendar was fixed by the Rabbis under the direction of Hillel the Second, only Heshvan can change to become 30 days and Kislev can lose a day to become 29 days. With the exception of Adar as noted above, the other months remain fixed.

The flexibility of these months is utilized to prevent Yom Kippur from falling adjacent to the Shabbat, because this would cause difficulties in coordinating the fast with Shabbat. Therefore a day is added to the month of Cheshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening.

Further Divisions of Time

The world is based on the division of the day into 24 equal hours. Each hour is sub-divided into 60 minutes and each minute is further sub-divided into 60 seconds. Each hour is exactly the same as any other hour. The day may have more hours in the summer than the night, but in the winter the nights have more hours than the days.

The Jewish calendar is different. In the Hebrew calendar the day also has 24 hours but they are not equal. The hours are divided into two groups, the day-hours and the night-hours. The days always have 12 hours and so do the nights. This means that the hours are not equal length. Sometimes on a long summer day a Jewish hour can be 72 minutes long, whereas on a winter day, the hour can only be 48 minutes long.

The subdivisions of the hour are different too. Each hour is divided into 1080 divisions, known as “chelakim” in plural or “chelak” in singular. This means that the minute has 18 “chelakim”. There are no further sub-divisions of the “minute”. It was considered that having the minute composed of 18 “chelakim” was sufficiently accurate to determine the time of the days and of the new moon.