JEWISH YEAR · 1 OF 3
Months and seasons
Our months follow the moon, while our festivals remain rooted in the seasons. Learn how those two rhythms belong to one Jewish year.
In this lesson
Two rhythms to keep together
A month is more than a page in a calendar. The changing appearance of the moon gives us a recurring cycle. A year brings another cycle: the movement of the seasons. The Jewish calendar keeps both in view.
Chodesh means “month.” Pesach, Passover, recalls our deliverance from slavery in Egypt. It comes in the springtime setting of the land of Israel, rather than moving freely through every season. To understand why, we will read the Torah and an explanation by Rambam, Rabbi Moshe ben Maimon.
The Torah is the five books of Moshe, Moses. Rambam lived many centuries later and organized Jewish law in his Mishneh Torah. One section of that code, the Laws of Sanctifying the New Moon, explains how our months and years are established.
Keep paper nearby. By the end, you will be able to explain a leap month and read a two-day Rosh Chodesh label. If day and night boundaries are still new, The Jewish day and week gives the starting map.
Moon for months, seasons for years
Rambam begins with two statements. Read them together:
חודשי השנה, הם חודשי הלבנה
The months of the year are lunar months.
והשנים שאנו מחשבין — הם שני החמה, שנאמר: שמור, את חודש האביב
The years we reckon are solar years, as it is said: “Keep the month of Aviv.”
English rendering by Messianic Yeshiva
Lunar means connected with the moon. Solar means connected with the sun; here it concerns the year's relation to the seasonal cycle. This combination is called lunisolar. The name is useful after we understand the two parts.
“Solar years” does not mean that every Jewish year has the same number of days as a civil-calendar year. Ordinary and leap years contain different numbers of lunar months. Rambam is explaining what keeps the year aligned with the seasons.
1. Which rhythm shapes the months, and which keeps the year in season?
The moon's cycle shapes the months. The solar seasonal cycle shapes the year's alignment. Keeping only the first sentence would miss Rambam's second requirement: our calendar holds the two together.
Why the season matters
Rambam supports his explanation with the Torah. In Deuteronomy, Moshe teaches our people as we prepare to enter the land of Israel:
שמור את חדש האביב ועשית פסח ליהוה אלהיך
Keep the month of Aviv, and observe Pesach for Hashem your God.
English rendering by Messianic Yeshiva
Hashem, “the Name,” is our customary way of referring to God. Aviv names the springtime of ripening grain. The rest of the verse connects this month with Hashem bringing us out of Egypt by night. Season and remembered deliverance belong together.
The seasonal setting is the land of Israel's. Our communities share a calendar across the world. Someone living where the local seasons are reversed does not move Pesach to another part of the Jewish year to match the weather outside.
Now a problem appears. Twelve lunar months are shorter than a solar seasonal year. If we simply repeated twelve months forever, a festival would gradually move through the seasons. Keeping the month of Aviv calls for an adjustment.
2. Why would twelve lunar months, repeated without adjustment, eventually move Pesach out of its spring setting?
Each twelve-month cycle would finish before the solar seasonal year had finished. That difference would accumulate, moving the festival earlier through the seasonal cycle. Naming the months alone would not keep the season aligned.
A leap month, not a leap day
וכמה יתרה שנת החמה על שנת הלבנה, קרוב מאחד עשר יום
How much does a solar year exceed a lunar year? Nearly eleven days.
מוסיפין חודש אחד; ועושין אותה השנה שלושה עשר חודש, והיא הנקראת שנה מעוברת
We add one month, making that year thirteen months; it is called a leap year.
English rendering by Messianic Yeshiva
In this comparison, “a lunar year” means twelve lunar months. The adjustment is not eleven extra days added to every year. From time to time, a whole month is added. The year then has thirteen months instead of twelve.
Rambam explains which month:
שנה מעוברת, היא שנה שמוסיפין בה חודש; ואין מוסיפין לעולם אלא אדר, ועושין אותה שנה שני אדרין — אדר ראשון ואדר שני
A leap year is a year in which a month is added. We add only Adar, making that year two Adars: Adar I and Adar II.
English rendering by Messianic Yeshiva
Adar is a month name. A leap year has Adar I and Adar II before the next Nisan, the month of Pesach. The added month keeps the festival cycle connected with its seasons. Our fixed calendar arranges seven leap years within a nineteen-year cycle; you do not need to calculate that cycle to follow a festival source.
3. “A Jewish leap year adds one day.” What needs changing?
It adds a month. There are two Adars and thirteen months in that year. A familiar civil calendar's leap day is a different adjustment; it should not replace the unit Rambam names.
A cycle and a numbered month
The average lunar cycle is a little more than twenty-nine and a half days. A calendar month, however, consists of whole days. Rambam distinguishes the two in Laws of Sanctifying the New Moon 8:1–3: our months contain either 29 or 30 days.
Imagine trying to make every month exactly thirty days. That would not keep matching the moon's cycle. But assigning a half-day at the end of each month is not how our numbered dates work either. The calendar uses whole-day months of two lengths.
Those lengths do not form a perfectly repeating short-long pattern in every year. Some month lengths vary under the fixed calendar. For now, remember the two possibilities rather than inventing a rule of alternation.
4. Does a lunar cycle of a little over 29½ days mean that the numbered month changes halfway through a calendar day?
No. The physical cycle and the calendar's whole-day count are related, but not identical. Calendar months contain 29 or 30 whole days. “Every month has thirty days” and “every month has a half-day date” both miss that distinction.
Who established the month?
Rambam describes an earlier setting in which an authorized beit din, a court of qualified judges, established the month. It was a communal decision, not a separate calendar made by each person who saw the moon.
אין ראיית הירח מסורה לכל אדם
Determining the month by seeing the moon is not entrusted to each individual.
אלא לבית דין הדבר מסור, עד שיקדשוהו בית דין ויקבעו אותו היום ראש חודש — הוא שיהיה ראש חודש
Rather, the matter is entrusted to the court: when the court sanctifies it and establishes that day as Rosh Chodesh, that is Rosh Chodesh.
English rendering by Messianic Yeshiva
Rosh Chodesh means “head of the month,” the beginning-of-month observance. In the following paragraphs, Rambam describes the court using calculations, examining witnesses and communicating its decision. Earlier determination involved both evidence and calculation.
5. In that earlier setting, did a private sighting by itself establish the month?
No. The authorized court examined the evidence and established the month. The excerpt concerns authority over the calendar, not a prohibition against looking at the sky. It also does not mean that our sages lacked calculation.
The received fixed calendar
Rambam then distinguishes that court setting from the calculated calendar. The Sanhedrin is the central rabbinic court discussed in his account:
ובזמן שאין שם סנהדרין, קובעין על חשבון זה שאנו מחשבין בו היום. ואין נזקקין לראייה
When there is no Sanhedrin, the months are established by the calculation we use today; observation is not required.
English rendering by Messianic Yeshiva
“Today” belongs to Rambam's explanation. Our received fixed calendar likewise determines the months and leap years by established calculation. We find our place in that shared calendar rather than wait for a personal sighting.
The first calendar date and the evening when a crescent becomes visible to a particular observer need not coincide. “The month has begun” and “I can see the moon from here” answer different questions.
6. A learner cannot see a crescent on the calendar's first day. Does that establish that the calendar is wrong?
No. Rambam distinguishes calculated dates from the requirement of observation. A particular person's visibility is not the test by which the fixed calendar begins its month. The source establishes that distinction without predicting what someone will see.
Why can Rosh Chodesh appear twice?
Rosh Chodesh may be observed for one day or two. Rambam explains the numbering in 8:4. After a 29-day month, the next day is the new month's first day and one day of Rosh Chodesh. After a 30-day month, Rosh Chodesh spans day 30 of the preceding month and day 1 of the new month.
Work through this illustration. Tishrei and Cheshvan are consecutive month names:
- 30 Tishrei
The last numbered day of Tishrei. It is also the first day of the two-day Rosh Chodesh observance.
- 1 Cheshvan
The first numbered day of Cheshvan. It is also the second day of that Rosh Chodesh observance.
The observance label extends across a boundary; the date number does not restart twice. This is an example of reading the labels, not a calendar for a particular year.
7. Which date begins Cheshvan's numbered month, and why can both dates say Rosh Chodesh?
1 Cheshvan begins its numbered month. The preceding 30 Tishrei shares the Rosh Chodesh observance label, but it remains a date in Tishrei. Distinguish the observance from the month and date written beside it.
Recall at your next session
Before reopening the lesson, explain a month, an ordinary year and a leap year. Then explain why a two-day Rosh Chodesh does not create two first dates, and why we do not set the calendar by a private sighting.
9. Check the five anchors.
A month follows the moon's cycle and contains 29 or 30 whole days. An ordinary year has twelve months. A leap year has thirteen, with two Adars, keeping the year in its seasonal setting. Two-day Rosh Chodesh spans the previous month's day 30 and the new month's day 1. Our fixed calendar establishes dates through received calculation.
Return to one source line that corrects a gap in your recall. Next we will ask a different question: which month is counted first?
Return to the sources
- Deuteronomy 16:1: Aviv, Pesach and deliverance from Egypt.
- Rambam, Laws of Sanctifying the New Moon 1:1–7: months, years and the court's work.
- 4:1 and 6:10–11: two Adars and the fixed nineteen-year cycle.
- 5:1–3: observation and fixed calculation.
- 8:1–6: whole-day months, Rosh Chodesh and month lengths.