🔑 Lesson Goals
By the end of this session, we will be able to:
1. Describe the historical conditions that led to the creation of a fixed calendar.
2. Identify the main components of the calculated Hebrew calendar.
3. Understand the basic purpose and function of dechiyot (postponement rules).
1. Scripture Quotes on the Calendar
1. Leviticus 23:4 - “These are the appointed times of the LORD, holy convocations which you shall proclaim at their appointed times.”
2. Numbers 10:10 - “Also in the day of your gladness… on your new moon and on your appointed feasts—you shall blow the trumpets over your burnt offerings…”
3. Deuteronomy 16:1 - “Guard the month of spring and observe the Passover to the LORD your God, for in the month of spring the LORD your God brought you out of Egypt by night.” 4. Isaiah 2:3 - “For from Zion will go forth the law, even the word of the LORD from Jerusalem.” 5. Exodus 12:2 - “This month shall be your beginning of months; it is to be the first month of the year to you.”
6. Deuteronomy 19:15 - “One witness shall not rise against a man for any iniquity… on the testimony of two or three witnesses the matter shall be confirmed.”
7. Mathew 18:16 - “But if he does not listen to you, take one or two more with you, so that BY THE MOUTH OF TWO OR THREE WITNESSES EVERY FACT MAY BE CONFIRMED.
8. 2 Corinthians 13:1 - This is the third time I am coming to you. EVERY FACT IS TO BE CONFIRMED BY THE TESTIMONY OF TWO OR THREE WITNESSES.
Historical Context (10–15 minutes)
1. Roman oppression and the breakdown of centralized Jewish authority in Eretz Yisrael.
2. Need for a universal calendar due to persecution, exile, and dispersion.
3. The end of calendar declarations by Sanhedrin or recognized courts.
4. Hillel II's decision (ca. 358–359 CE) to publish rules for calendar calculation.
Sources:
1. Babylonian Talmud, Rosh Hashanah 20a–21a
“Some say that Rav Naḥman said:… We learn… witness‑based sanctification is valid…” demonstrating the early requirement for witness testimony before calculation became universal.
Sefaria+1Ask The Rabbi - Dinonline+1
2. Letter of R. Hai Gaon (10th c.) regarding the calendar and its publication According to Hai Gaon, the fixed calendar was instituted by Hillel II in the year 4119 AM
(358/359 CE), when he “published the rules of intercalation for all communities in exile.” asif.co.il+4Wikipedia+4hakirah.org+4
3. Rambam’s Mandate (Mishneh Torah, Kiddush HaChodesh ch. 1) “It contains one positive commandment: To calculate, to determine and to establish the day on which each of the months of the year will begin.” Chabad+1The Rabbinical Assembly+1
2. Structure of the Hebrew Calendar (10 minutes)
(a) Lunisolar system: combining lunar months with the solar year.
(b) Months: either 29 and 30 days to average the lunar month.
(c) Year types:
a. Regular year: 12 months (353–355 days).
b. Hebrew term: שנה פשוטה (shana peshutah)
Meaning: "Simple year" or "ordinary year"
i. Contains 12 lunar months
ii. Approximately 354 days (depending on the length of months)
iii. Does not include an extra Adar (leap month)
a. Leap year
i. Hebrew term: שנה מעוברת (shana meʿuberet)
Literal meaning: "Pregnant year" (as if it is “swollen” with an extra month)
ii. Contains 13 lunar months
iii. Adds Adar II (Second Adar) after Adar I
iv. Approximately 384 days
3. Cycle System and Month Start (5 minutes)
1. 19-year Metonic cycle: seven leap years (years 3, 6, 8, 11, 14, 17, 19).
2. Calculation of the molad (mean conjunction) to establish Rosh Chodesh.
3. Traditionally a day begins at sundown, molad traditionally at noon.
4. Introduction to Dechiyot (Postponement Rules) (10 minutes)
Purpose: prevent undesirable weekday alignments for major holidays.
Summary Table
Dechiyah
Name
Reason
Effect
Lo Adu Rosh
Rosh Hashanah can't be
on Sun א/Wed ד/Fri ו
Practical observance of
Yom Kippur and
Hoshanah Rabbah
Delays Rosh Hashanah by 1
day if calculated on these days.
Molad Zaken
Late Molad
Rosh Hashanah
shouldn't precede the
new moon
If molad is after noon, Rosh
Hashanah is postponed 1 day.
Gatarad
3rd Dechiyah
Year length correction
Postpones Rosh Hashanah
from Tuesday to Thursday.
Badach
4th Dechiyah
Leap year length
correction
Postpones Rosh Hashanah
from Monday to Tuesday or
Thursday.
5. Dechiyot (Postponements) Explanation Quotes
“If the molad occurs at or later than noon, Rosh Hashanah is postponed… This is called deḥiyyat molad zaken… and if the molad occurs on a Sunday, Wednesday, or Friday, Rosh Hashanah is postponed… called deḥiyyat lo ADU… Additional rules include GaTaRaD and BeTUTeKaPoT to prevent impermissible placements of holidays.” Shulchan Aruch Harav+2The Rabbinical Assembly+2Wikipedia+2Chabad+3Wikipedia+3The Rabbinical Assembly+3
6. Why was it important to publish a fixed calendar?
1. Preservation of Unity Among the Jewish People
a. A single, published calendar ensures that Jewish communities around the world observe festivals and holy days on the same dates.
b. "For from Zion will go forth the law, And the word of the LORD from Jerusalem" (Isaiah 2:3, NASB 1995)
c. Even though the law originates from Zion, a calculated and published calendar allows that law to be followed uniformly across the diaspora.
2. Practicality in the Diaspora
After the destruction of the Second Temple and especially following the exile, reliance on eyewitnesses reporting the new moon to the Sanhedrin in Jerusalem was no longer possible.
The calculated calendar enabled Jews in far-off lands to know the dates in advance.
Rav Saadia Gaon wrote:
“Were it not for the calculated calendar, Jews outside of the Land of Israel would not be able to observe the festivals on the proper days... The unity of Israel depends upon a unified calendar.”
(Source: Sefer ha-Mo'adim)
3. Avoidance of Religious Confusion
a. A published calendar avoids conflicts and divisions that might arise from multiple observance days in different communities.
b. Babylonian Talmud, Rosh Hashanah 25a:
“Rabban Gamliel said: I have it on the authority of my grandfather’s court that the new moon is to be sanctified even if it is seen only slightly... one must accept the ruling of the court.
c. This shows the importance of having a central authority to establish the calendar and avoid confusion.
4. Ensures Continuity of Torah Commandments
a. By enabling timely observance of Moedim (appointed times), the published calendar helps preserve obedience to the Torah in practice.
b. Leviticus 23:4 (NASB95)
“These are the appointed times of the LORD, holy convocations which you shall proclaim at the times appointed for them.”
Without a known calendar, these could not be properly “proclaimed.”
5. Protection from Persecution
a. During times of Roman and later Islamic or Christian oppression, openly declaring the new moon became dangerous. A secret or pre-calculated calendar helped Jewish communities avoid persecution.
b. Letter of Hai Gaon (10th century CE):
“We rely on the calculation alone, since we are scattered and no longer have a court to sanctify the moon. The calendar is publicized for the sake of preserving Torah in all lands.”
6. Adaptability for Legal and Communal Life
a. A fixed calendar allows rabbinical courts to plan for fasts, holidays, legal rulings, and the reading cycle of Torah and Haftarah portions far in advance.
b. Babylonian Talmud, Sanhedrin 12a:
“They would calculate the seasons and the intercalation of the year, in order that the festivals might fall in their proper seasons.”
This underscores the legal importance of accurate calendrical forecasting.
📈 Advantages of the Calculated Calendar
1. Predictability and Stability
All dates for holidays and fasts can be calculated years in advance, allowing for global planning and consistent observance.
2. Universal Uniformity
Communities in the Diaspora and in Israel observe festivals on the same days, unifying Jewish practice worldwide.
3. No Need for Witness Testimony
Removes reliance on eyewitness reports for new moon sightings, which can be unreliable due to weather, geography, or political instability.
4. Resilient During Exile and Persecution
Ensured calendar continuity when Sanhedrin authority ended and observatories in Jerusalem were no longer accessible.
5. Legal Finalization of Festival Dates
Ensures that even if natural signs (e.g., barley ripening or clear skies) fail, the festivals are fixed according to halakhic authority.
⚠️ Limitations of the Calculated Calendar
1. Detachment from Natural Cycles
The fixed calendar can place Passover before the barley is ripe or before the spring equinox in some years, contrary to Deuteronomy 16:1.
2. Drift from Original Torah Timing
Because it relies on mathematical averages and not actual sightings, it can misalign with the biblical “appointed times” (,מועדים mo’edim).
3. Lack of Local Adaptability
Conditions in Israel may differ from calendar calculations made centuries ago in Babylonia;
barley and moon sighting may no longer match the set dates.
4. No Sanhedrin Oversight
a. The original Torah calendar was managed by an authorized court. Without a functioning Sanhedrin, the fixed calendar lacks that foundational authority.
b. Every man does what is right in their own eyes.
5. Disputes over Validity
a. Karaites, some Messianic groups, and others challenge the authority or correctness of the fixed calendar, especially when it visibly diverges from observable signs.
b. Most of this revolves around the dispute over who has the authority in these matters.
Hopefully I have given you enough information that you have at least a basic knowledge of how the Hillel II calendar works and why we use it here at Roeh Israel – Abilene.
Next week I wish to examine the original calendar basedon sighting of the waxing crescent and other agricultural signs.
HaSof
