What Are the Basic Rules of the Jewish Calendar?

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Thirty Days has September, April, June, November, Tishrei, Shvat, Nissan, Sivan, Av and sometimes Cheshvan[1] and Kislev. Yet a reading of Mishnah Rosh Hashanah implies that whether a month has 29 days or 30 depends on when the witnesses saw the new moon and testified in Beis Din early enough to declare the thirtieth day Rosh Chodesh. In addition, the Gemara[2] notes that Elul could be thirty days long, something that cannot happen in our calendar. How did our empirical calendar become so rigid and predictable in advance? Come with me as we explore the history and foundations of the Jewish calendar!

The Torah (Shemos 12:2) commands the main Beis Din of the Jewish people, or a Beis Din specially appointed by them, to declare Rosh Chodesh upon accepting the testimony of witnesses who observed the new moon (Rambam, Hilchos Kiddush HaChodesh 1:1, 7; 5:1). The purpose of having eyewitnesses was not to notify the Beis Din of its occurrence; the Beis Din had extensive knowledge of astronomy and already knew exactly when and where the new moon would appear and what size and shape it would have (Rambam, Hilchos Kiddush HaChodesh 2:4; Ritva on the Mishnah Rosh Hashanah 18a). The moon’s location and speed is constantly influenced by many factors, but the wise scholars of the tribe of Yissachar calculated where and when it would appear.

The Molad

As the moon orbits earth, we on earth observe it as passing through its various phases, from the very smallest crescent until full moon, and then shrinking until it disappears completely. This monthly cycle occurs because the moon has no light of its own, and only reflects sunlight back to earth. As the moon travels around the earth, the angle at which it reflects light changes. This evidences itself in the moon’s changing phases. When the moon is on the side of the earth away from the sun, we see the full moon, because it is now at an angle whereby the entire side is reflecting light to us. However, when the moon is on the side of earth nearest the sun, we see no reflection of its light at all, and that is the point of every month when the moon disappears from earthly view. The molad is the point at which the moon crosses the plane between the earth and the sun, which means it is the beginning of a new cycle, called in English the new moon and in technical jargon the point of conjunction.

From the time of the actual molad you can calculate when the moon will become visible. Chazal always kept secret how one can predict when the new moon was to appear so as to avoid false witnesses abusing the knowledge of this information (Rambam, Hilchos Kiddush HaChodesh, 11:4).

The purpose of having eyewitnesses was not to notify the Beis Din of its occurrence; rather, the Torah required the Beis Din to wait for witnesses to determine whether the 30th day (of the previous month) would be the last day of the old month or the first day of a new month. If no witnesses to the new moon testified on the 30th day, then the new month does not begin until the 31st day, regardless of the astronomic calculations (Mishnah Rosh Hashanah 24a). Thus, prior to the establishment of our current “permanent” calendar, any month could be either 29 or 30 days, dependent on when the new moon appeared and whether witnesses arrived in Beis Din to testify about this phenomenon.

By the way, we should be aware that the above description follows the opinion of the Rambam, that the preferred and original mitzvah is to declare Rosh Chodesh based on the testimony of witnesses. However, there are several early authorities, including Rav Saadyah Gaon and Rabbeinu Chananel, who hold that the primary mitzvah is to declare Rosh Chodesh on the basis of the calculations, and that use of witnesses was implemented because of certain circumstantial issues.

According to either approach, the calendar printers could not go to press until the Beis Din had declared Rosh Chodesh, which probably explains why calendar manufacture in those times was a difficult business in which to turn a profit. Perhaps this is why organizations mailed out so few fundraising calendars in the days of Chazal!

There is another commandment of the Torah – that Pesach must always occur in the Spring (Devarim 16:1). This seemingly innocuous obligation actually requires considerable manipulation of the calendar, since the months, derived from the word moon, are determined by the length of time from one new moon to the next, which is a bit more than 29 1/2 days. However, the year and its seasons are determined by the relative location of the sun to the earth, which is a bit less than 365 1/4 days. By requiring Pesach to always be in the spring, the Torah required that the calendar could not be exclusively twelve lunar months, since this would result in Pesach wandering its way through the solar year and occurring in all seasons.[3]

Among contemporary calendars, most make no attempt to accommodate the solar year and the lunar month. What we refer to as the common secular calendar, or the Gregorian calendar, is completely based on the sun. Although the year is broken into months, the use of the word “months” is borrowed from its original meaning and has been significantly changed since the months have no relationship to any cycle of the moon. Most of the secular months have 31 days, while the lunar cycle is only about 29 1/2 days, and even those secular months that have 30 days do not relate to any phase or change in the moon. Similarly, the length of February as a month of either 28 or 29 days has nothing to do with the moon. Thus, although the word month should correspond to the moon, the Western calendar is purely a solar one, with a borrowed unit “month” given a meaning that distorts its origins.

The Moslem calendar is purely a lunar calendar of twelve lunar months, some 29 days and some 30, but has no relationship to the solar year. In truth, a pure lunar calendar has no real “year,” since a year is based on the relative locations of the sun and the earth and the resultant seasons, and the Moslem year completely ignores seasons. The word “year” is used in the Moslem sense only as a basis for counting longer periods of time, but has no relationship to the sun. Thus the Moslem “year” is only 354 or 355 days long — almost 11 days shorter than a true solar year. Therefore, a Moslem who tells you that he is 65 years old is really closer to 63 according to a solar year count. He has counted 65 years that are at least ten days shorter. I trust that Guinness takes these factors into account when computing longevity, and insurance companies realize this when calculating actuarial tables.

To review: the Moslem calendar accurately tracks the moon and the months, but has no relationship to a true year, and the Western secular calendar is fairly accurate at tracking the year and its seasons, but has no relationship to the moon and its phases.

It is noteworthy that although the Moslem “year” does not correspond at all to a solar or western year, it closely corresponds to our Jewish year in a “common” year which is only twelve months long, and the Moslem month follows closely the Jewish calendar month. (We will soon explain why there is sometimes a discrepancy of a day or two.) Thus, for three years recently, Ramadan, the Moslem holy month, corresponded to our month of Elul, although this year Ramadan falls in Av. It is accurate to say that the Moslem year “wanders” its way through the seasons as it takes 33 years until a specific month returns to the same corresponding time in the solar year, and in the interim the month has visited each of the other seasons for several consecutive years. Thus, Ramadan will not coincide with Elul again this generation, but falls in Av for three years, with Tamuz for two years, and then with Sivan, etc.

However, when Hashem commanded us to create a calendar, He insisted that we use the moon to define the months, and yet also keep our months in sync with the seasons, which are dependent on the sun; to determine the dates of the Yomim Tovim. The only way to do this is to use the Jewish calendar method of occasionally adding months – thereby creating 13 month years, which we call “leap years,” to offset the almost 11 day difference between twelve lunar months and a solar year. The result of this calendar is that although each date does not fall exactly on the same “solar date” every year, it falls within a fairly close range relative to the solar year.

Who determined which year has thirteen months?

The original system was that the main Beis Din (also known as the Sanhedrin) appointed a smaller special Beis Din to determine whether the year should have an extra month added. This special Beis Din took into consideration:

1) Astronomical data, such as: When Pesach will fall out relative to the vernal equinox (the Spring day on which day and night are closest to being equal in length).

2) Agricultural data, such as: How ripe is the barley? How large are the newborn lambs and pigeons?

3) Weather: Is the rainy season drawing to a close? Is it a famine year?

4) Convenience – or more specifically, the halachic inconvenience of creating a leap year: Shemittah was never made into a leap year, and the year before shemittah usually was.

5) Infrastructure, such as: In what condition were the highways and bridges.

All of these points influenced whether the thirteenth month, the extra Adar, would be added.[4] When this system was in place, which was from the time of Moshe and Yehoshua until almost three hundred years after the destruction of the Beis Hamikdash, the main Beis Din sent written messages notifying outlying communities of the decision to create a leap year and the reasons for their decision.[5]

By the way, after the destruction of the Beis HaMikdash, the main Beis Din was not located in Yerushalayim, but wherever the Nasi of the Jewish people resided, as long as it was in Eretz Yisrael. This included several other communities at various times of Jewish history, including Teverya, Yavneh, and Shafraam.[6] Indeed, during this period sometimes the special Beis Din met outside the land of Israel — should the head of the Beis Din be in the Diaspora and there be no one of his stature remaining in Eretz Yisrael.[7]

This explains how the calendar is intended to be calculated.  Why and how our current calendar came to be will be discussed in a future article.


[1] Although the correct name of the month is Marcheshvan, we will follow the colloquial use of calling it Cheshvan.

[2] Rosh Hashanah 19b, 20a

[3] Rambam, Hilchos Kiddush HaChodesh 4:1

[4] Sanhedrin 11a- 12a

[5] Sanhedrin 11b; Rambam, Hilchos Kiddush HaChodesh 4:17

[6] Rosh Hashanah 31b

[7] Berachos 63a; Rambam, Hilchos Kiddush HaChodesh 1:8

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