The Evolution of the Hebrew Calendar

Hebrew Months

By Gideon

The Jewish calendar is used for religious purposes by Jews all over the world, and it is the official calendar of Israel. A Jewish-calendar day does not begin at midnight, but at either sunset or when three medium-sized stars should be visible, depending on the religious circumstance. Sunset marks the start of the 12 night hours, whereas sunrise marks the start of the 12 day hours. This means that night hours may be longer or shorter than day hours, depending on the season. Years are counted since the creation of the world, which is assumed to have taken place in the autumn of 3760/3761 B.C.E.  The current Jewish year is 3760 +2016= 5776

The study of astronomy was largely due to the need of fixing the dates of the festivals. The command Deuteronomy Chapter 16.1 made it necessary to be acquainted with the position of the sun.

Deuteronomy Chapter 16.1 דְּבָרִים

א שָׁמוֹר, אֶת-חֹדֶשׁ הָאָבִיב, וְעָשִׂיתָ   פֶּסַח, לַיהוָה אֱלֹהֶיךָ: כִּי בְּחֹדֶשׁ הָאָבִיב, הוֹצִיאֲךָ יְהוָה   אֱלֹהֶיךָ מִמִּצְרַיִם–לָיְלָה. 1 Observe the month of Abib, and keep the passover   unto the LORD thy God; for in the month of Abib the LORD thy God brought thee   forth out of Egypt by night.

The Jewish calendar must meet two requirements, both solar and lunar. This accounts for its relatively complicated structure. Since the solar year of about 365 days is approximately 11 days longer than 12 lunar months, the Jewish calendar is faced with the problem of balancing the solar with the lunar years. The traditional law prescribes that the months shall follow closely the course of the moon, from its Molad (birth, conjunction) to the next New Moon. The lunar months must always correspond to the seasons of the year, which are governed by the sun. The month of Nisan with the Passover Festival, for instance, must occur in the spring and the month of Tishri with the harvest festival of Succoth in the fall.

In the early times the beginnings of the months were determined by direct observation of the new moon. Then those beginnings of the months (Rosh Hodesh) were sanctified and announced by the Sanhedrin, the Supreme Court in Jerusalem, after witnesses had testified that they had seen the new crescent and after their testimony had been thoroughly examined, confirmed by calculation and duly accepted.

Originally, the starting date of every new month was broadcast from Jerusalem to distant Jewish outposts via huge bonfires which would be lit on designated mountaintops. Lookouts stationed on other mountaintops would see that a fire had been lit and would light their own fire. This chain of communication led all the way to Babylon, and even very distant communities knew when the new month began. If there was a festival that month, they now knew when to celebrate it. But a problem arose. The Sadducees, a sect of Jews who denied rabbinic authority and were constantly at odds with the Sanhedrin, started lighting fires on the wrong days in order to manipulate the calendar.

To prevent this confusion, the fire-on-mountaintop method of communication was discontinued, and instead messengers were dispatched to Babylon and all other far-flung Jewish settlements. Since news travels a lot slower that way, distant communities would not know when Rosh Chodesh (the “Head of the Month”) had been declared in time to celebrate the festival on the proper day. It was therefore decreed that outside of the Land of Israel people would celebrate two days for each festival – the day it would be if the previous month had been a 29 day month, and the day it would be if the previous month had been a 30 day month. Even though nowadays we have a fixed calendar, this tradition continues. Jews outside the Land of Israel celebrate each biblical festival for two days. There are two exceptions to this rule:

1) Yom Kippur is still only celebrated for one day because fasting for two days is untenable.

2) Rosh Hashanah is celebrated for two days even in the Land of Israel. Rosh Hashanah is celebrated the first day of the month of Tishrei. Until Rosh Chodesh Tishrei was declared, communities both inside and outside of the Land of Israel were subject to the same uncertainty as to when to celebrate Rosh Hashanah. So everyone everywhere celebrated it for two days.

A special committee of the Sanhedrin, with its president as chairman, calculated the beginnings of the seasons (Tekufoth) on the basis of astronomical figures which had been handed down as a tradition of old. Whenever, after two or three years, the annual excess of 11 days had accumulated to approximately 30 days, a thirteenth month Adar II was inserted before Nisan in order to assure that Nisan and Passover would occur in Spring and not retrogress toward winter.

However, the astronomical calculation was not the only basis for intercalation of a thirteenth month. The delay of the actual arrival of spring was another decisive factor. The Talmudic sources report that the Council intercalated a year when the barley in the fields had not yet ripened, when the fruit on the trees had not grown properly, when the winter rains had not stopped, when the roads for Passover pilgrims had not dried up, and when the young pigeons had not become fledged. The Council on intercalation considered the astronomical facts together with the religious requirements of Passover and the natural conditions of the country. This method of observation and intercalation was in use throughout the period of the second temple (516 B.C.E – 70 C.E), and about three centuries after its destruction, as long as there was an independent Sanhedrin.

In the Torah, the beginning of the year was set at 1 Nisan, in the context of a description of the first Passover. “The Lord said to Moses and Aaron in the land of Egypt: This month shall mark for you the beginning of the months; it shall be the first of the months of the year for you” (Exodus 12:1-2). This new year celebrated the creation of the Jewish nation through the redemption of the Israelites from Egypt. Nisan, as the first of the months, coincided with the beginning of Jewish national history. Yet by the period of the Mishnah at the beginning of the second century, the outlines of today’s Rosh Hashanah holiday are clear; and discussions about the prayers of Rosh Hashanah appear as early as the teachings of the schools of Hillel and Shammai, which date to the first century CE. Mishnah Rosh Hashanah 1:1 specifically defines Rosh Hashanah’s “new year” status. “The first of Tishrei is the beginning of the year [rosh hashanah] for years, sabbatical cycles, and the jubilee.” Although the functions of this new year relate primarily to the agricultural cycle and the beginning of a new harvest year, the Mishnah also begins to assign to it conceptual and theological meaning. Sometime between the Torah and the codification of the Mishnah, the autumn new year gained ascendance, now transformed into a major celebration, and the Nisan new year was left as a marker of the months and festivals in the calendar year. [How Rosh Hashanah Became New Year’s Day]

One of the important figures in the history of the calendar was Samuel (born about 165, died about 250), surnamed “Yarḥinai” because of his familiarity with the moon. He was an astronomer, and it was said that he knew the courses of the heavens as well as the streets of his city (Ber. 58b). He was director of a school in Nehardea (Babylonia), and while there arranged a calendar of the feasts in order that his fellow-countrymen might be independent of Judea. He also calculated the calendar for sixty years. His calculations greatly influenced the subsequent calendar of Hillel. According to Bartolocci his tables are preserved in the Vatican. A contemporary of his, R. Adda (born 183), also left a work on the calendar. Mar Samuel reckoned the solar year at 365 days and 6 hours, and Rab Adda at 365 days, 5 hours, 55 minutes, and 25 25/57 seconds.

In 325 the Council of Nice was held, and by that time the equinox had retrograded to March 21. This council made no practical change in the existing civil calendar, but addressed itself to the reform of the Church calendar, which was soli-lunar on the Jewish system. Great disputes had arisen as to the time of celebrating Easter. Moreover, the Church was not fully established, many Christians being still simply Jewish sectarians. A new rule was therefore made, which, while still keeping Easter dependent on the moon, prevented it from coinciding with Passover.

Under the patriarchate of Rabbi Judah III. (300-330) the testimony of the witnesses with regard to the appearance of the new moon was received as a mere formality, the settlement of the day depending entirely on calculation. This innovation seems to have been viewed with disfavor by some members of the Sanhedrin, particularly Rabbi Jose, who wrote to both the Babylonian and the Alexandrian communities, advising them to follow the customs of their fathers and continue to celebrate two days, an advice which was followed, and is still followed, by the majority of Jews living outside of Israel.

Under the reign of Constantius (337-361) the persecutions of the Jews reached such a height that all religious exercises, including the computation of the calendar, were forbidden under pain of severe punishment. The Sanhedrin was apparently prevented from inserting the intercalary month in the spring; it accordingly placed it after the month of Ab (July-August).

In the fourth century, when oppression and persecution threatened the continued existence of the Sanhedrin, the patriarch Hillel II, the Sanhedrin president, in approximately C.E. 359, took an extraordinary step to preserve the unity of Israel. In order to prevent the Jews scattered all over the surface of the earth from celebrating their New Moons, festivals and holidays at different times, he made public the system of calendar calculation which up to then had been a closely guarded secret. It had been used in the past only to check the observations and testimonies of witnesses, and to determine the beginnings of the spring season. In accordance with this system, Hillel II formally sanctified all months in advance, and intercalated all future leap years until such time as a new, recognized Sanhedrin would be established in Israel. This is the permanent calendar according to which the New Moons and Festivals are calculated and celebrated today by the Jews all over the world. Like the former system of observation, it is based on the Luni-Solar principle. It also applies certain rules by which the astronomical facts are combined with the religious requirements into an admirable calendar system.

An ordinary (non-leap) year has 353, 354, or 355 days. A leap year has 383, 384, or 385 days. The three lengths of the years are termed, “deficient,” “regular,” and “complete,” respectively.

The months of the Jewish calendar are referred to mostly by number in the Bible, but were also given names almost identical to the names for the Babylonian months. The Talmud (Yerushalmi, Rosh ha-Shanah i. 1) correctly states that the Jews got the names of the months at the time of the Babylonian exile.

1.Nissan 2. Iyar 3. Sivan 4. Tamuz 5. Av 6. Elul 7. Tishrei 8. Cheshvan 9. Kislev 10. Tevet 11. Shevat 12. Adar I  13. (Adar II)

An ordinary year has 12 months; a leap year has 13 months.

Every month starts (approximately) on the day of a new moon.

The months and their lengths are:

Name Length   in a deficient year Length   in a regular year Length   in a complete year
Tishri 30 30 30
Heshvan 29 29 30
Kislev 29 30 30
Tevet 29 29 29
Shevat 30 30 30
Adar   I 30 30 30
Adar   II 29 29 29
Nisan 30 30 30
Iyar 29 29 29
Sivan 30 30 30
Tammuz 29 29 29
Av 30 30 30
Elul 29 29 29
Total: 353   or 383 354   or 384 355   or 385

In a regular year the numbers 30 and 29 alternate; a complete year is created by adding a day to Heshvan, whereas a deficient year is created by removing a day from Kislev.

The alteration of 30 and 29 ensures that when the year starts with a new moon, so does each month.

With the exception of the Shabbat, the weekdays have no names.  They are simply numbered:

yom rishon = “first day” = (Sunday)

yom sheni = “second day” = (Monday)

yom sh’lishi = “third day” = (Tuesday)

yom revi’i = “fourth day” = (Wednesday)

yom chamishi = “fifth day” = (Thursday)

yom shishi = “sixth day” = (Friday)

The week culminates in the seventh day, the Holy Shabbat

According to the Jewish calculation, creation was in the autumn of 3760/ 3761 BCE, slightly later than the traditional Christian figure of 4004 BCE. Thus the year 5701 began in 1940 in the Gregorian calendar and ended in 1941. Using that as a starting point, it is easy to convert years between the two calendars, remembering always that the new years do not coincide so a year in one calendar will always overlap with two years in the other. A year is a leap year if the remainder (in AM) on division by 19 is 3, 6, 8, 11, 14, 17 or 0. Thus 5703, 5706, 5708, 5711, 5714, 5717 and 5719 were leap years with 13 months each, but not the years between, which only had 12 months. This rule is based on the Metonic cycle, which assumes that 19 years exactly equal 235 lunar months. The 19 year cycle is quite accurate. Assuming 365.24219 days for the tropical year, to do better with a fixed cycle would require a cycle of 182 years containing 2,251 months. However, it is not perfect.


Jewish month

Civil date of first day of Jewish month

Length of year

353 days 354 days 355 days 383 days 384 days 385 days


23 March 23 March 23 March 23 March 23 March

23 March


22 April 22 April 22 April 22 April 22 April

22 April


21 May 21 May 21 May 21 May 21 May

21 May


20 June 20 June 20 June 20 June 20 June

20 June


19 July 19 July 19 July 19 July 19 July 19 July


18 August 18 August 18 August 18 August 18 August

18 August


16 September 16 September 16 September 16 September 16 September

16 September


16 October 16 October 16 October 16 October 16 October

16 October


14 November 14 November 15 November 14 November 14 November

15 November


13 December 14 December 15 December 13 December 14 December

15 December


11 January 12 January 13 January 11 January 12 January

13 January

Added month

10 February 11 February

12 February


10 February 11 February 12 February 12 March 13 March

14 March

First 11 March 12 March 13 March 10 April 11 April 12 April