The Biblical Calendar

Joshua Perez

1/1/2025

History testifies that there are two calendars that were observed by the Jewish people. In this article we will explore each of them, and explain which one she be kept today.

The Ancient Jewish Calendar 

The first calendar we will cover is "The Ancient Jewish Calendar," which was based on specific instructions given in the Torah, and was observed during the First and Second Temple period. This calendar reckons the beginning of its months by the sighting of the first observable crescent of the moon (approximately 2% illumination) [1]. In ancient Israel, witnesses would watch the skies around sunset on the 29th and 30th days of each month, waiting to observe the new moon. If sighted, they would present themselves before the Sanhedrin. If their testimony was found to be accurate, the Sanhedrin would declare that a new month had begun [2].

This calendar also calculates the addition of a leap year (Adar II, the 13th month) based on several factors, but primarily the maturation of agriculture in the land of Israel, specifically the barley crop. People living in the land of Israel would check the barley crop during the last few days of the 12th month to see if it had reached the state of ripeness called "Aviv" [3]. If the barley had reached this stage, the month of Aviv and the new year could begin. If the barley had not reached this state of ripeness, a 13th month would be added to allow the crop more time to mature. This method results in the addition of a 13th month approximately once every three to four years, solely contingent on the maturation of the barley crop [4]. 

The final component of this calendar is the seventh-day Sabbath, which is observed from Friday evening to Saturday evening (see our article "The Sabbath Day" for more). The Saturday Sabbath is the one component of this calendar that is also shared with its modern counterpart, which we will now cover.

The Modern Jewish Calendar 

The second calendar we will cover is "The Modern Jewish Calendar," created in the 4th century AD (358 AD) by Hillel II as one of the last official acts of the Sanhedrin before it fully disbanded. This calendar aimed to mathematically and astronomically fix the dates of the various feast days to unify Jews in the diaspora, who otherwise might have observed different dates based on local moon sightings.

"The fixed calendar was established by Hillel II in the 4th century C.E. to ensure uniformity among Jewish communities. It replaced the earlier system of relying on witnesses to observe the new moon, which had become impractical due to the political climate of the time." (The Jewish Encyclopedia - Calendar)

"The calendar of Hillel II, established in the 4th century C.E., was a significant turning point in Jewish history. It was designed to avoid the confusion caused by regional differences in moon sighting and to provide a uniform method for all Jews to determine the dates of festivals and holidays." (Encyclopedia Judaica - Calendar)

"Hillel II’s calendar, established in approximately 358 C.E., standardized the Jewish calendar based on mathematical calculations rather than lunar observations, which had become increasingly difficult to maintain due to the diaspora and political challenges." (The Encyclopedia of Judaism - Calendar)

This calendar, unlike its ancient counterpart, calculates the beginning of its months based on the conjunction of the moon (0% illumination), rather than the first observable crescent. 

"The Jewish calendar determines the beginning of the month by the new moon, which is identified astronomically with the conjunction of the moon and sun. The months are based on lunar cycles, and the calendar has been adjusted to align with solar years." (The Jewish Encyclopedia - Calendar)

The final addition to this calendar was the mathematical interpolation of leap years, eliminating the need to observe crop maturation in the land of Israel. To achieve this, the calendar adds a 13th month (Adar II) seven times over a 19-year cycle. This ensures that Jews everywhere can remain aligned on a unified calendar, without the necessity of being present in the land of Israel.

"The Jewish calendar is based on lunar months, with each month beginning on the new moon. The calendar employs a system of intercalation, adding a 13th month in seven out of every 19 years, to keep the calendar aligned with the solar year." (The Jewish Encyclopedia - Calendar)

"The Jewish calendar follows a 19-year cycle (the Metonic cycle), during which seven leap years occur, wherein a 13th month (Adar II) is added. This system allows the lunar calendar to remain in sync with the seasons." (Encyclopaedia Judaica - Calendar)

Since this calendar also observes Friday evening to Saturday evening as the Shabbat, we again recommend reading our article "The Sabbath Day" for further details. According to the Torah, when Moses bestowed authority upon the council of judges (The Sanhedrin), it granted them the power to establish halachah (legal rulings on Written and Oral Law). This authority enabled them to devise a new calendrical system if necessary.

"You shall appoint judges and officers in all your towns that the Lord your God is giving you, according to your tribes, and they shall judge the people with righteous judgment." (Deuteronomy 16:18)

"If any case arises which is too hard for you to judge, whether it concerns bloodshed, lawsuits, or assaults, matters of dispute in your towns, then you shall arise and go up to the place that the Lord your God will choose. And you shall come to the Levitical priests and to the judge who is in office in those days, and you shall consult them, and they shall declare to you the decision. Then you shall do according to what they declare to you from that place that the Lord will choose. And you shall be careful to do according to all that they direct you. According to the instruction that they give you, and according to the decision which they pronounce to you, you shall do; you shall not turn aside from the verdict that they declare to you, either to the right or to the left." (Deuteronomy 17:8-13)

The council of 70 elders and judges evolved into what is now commonly referred to as the Great Sanhedrin, which, according to the Torah, was endowed with the authority to create what we now recognize as the Modern Jewish Calendar. The Messiah Yeshua himself affirmed the Sanhedrin's authority, when he recognized that they sit in the seat of Moses, and stated that whatever they instruct to observe must be done carefully (Matthew 23:1-3). For a comprehensive overview of the Sanhedrin's authority, please refer to our article "The Sanhedrin."

Due to this ruling, we recognize the Hillel II calendar as valid according to Torah, and believe that following it is the appropriate way to fulfill the mitzvot of keeping Hashem's sabbaths holy today. For a simple resource to observe this calendar, we recommend visiting the Chabad website: 

https://www.chabad.org/holidays/default_cdo/year/2024/jewish/holidays-2024.htm

[1] Philo, Special Laws II, 141, p.581, Hendrickson, 1997;

[2] Rosh Hashannah 22a

[3] Exodus 9:31-32, 13:4, 34:18; Deuteronomy 16:1

[4] Sanhedrin 11b