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Did you ever wonder why Jewish holidays fall on different dates every year?

In order to answer that question we must start with the first Mitzvah given to the Bnei Yisroel - The Mitzvah of Kiddush Ha-Chodesh, (Sanctification of the New Moon or beginning of the new month).

Kiddush Ha-Chodesh is the first Mitzva given to the Bnei Yisroel. (didn't I just say that). In Shmot, Parshat Bo, Moshe declares in Hashem's name, "This month (NISAN) shall be to you the first month:

What? NISAN the first month? You probably thought it was TISHREI! Isn't that when we observe Rosh Hashana? Well, you're partially correct.

Just like school starts in September and some businesses start their schedule (fiscal) year in June, the Jewish year has different starting points too. The Birth of the World and when Hashem judges the world, is in TISHREI. For "Ma'asering" (tithing) fruits of the tree, it is the 15th of Shvat (Tu B'Shvat). And now, in recognition of Yetziat Mitzraim, (the Exodus from Egypt), Hashem wants the Bnei Yisroel to mark NISAN on their calendars as the first month of the year for the newly freed Bnei Yisroel.

This is very important to know because in the Torah, the months are never referred to by their proper names. They are always referred to as "the first month" ..(NISAN), "the seventh month" ..(TISHREI), and so on. (We'll talk more about the names of the months later).

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How is the Rosh Chodesh, (beginning of the month), of each month determined?

Look at the calendar, you say?

Wrong!! When the Bnei Yisroel reach Eretz Yisroel, they are to appoint a Sanhedrin - a court of learned men. This body of leaders will decide each month which day Rosh Chodesh will be.

The Jewish calendar is based on the cycle of the moon. If you've ever observed the moon in the evening sky, this heavenly body starts out looking like a very thin half circle and ultimately grows to the full circle in the middle of the month. Then it shrinks again to a sliver. These changes are called "the phases of the moon." At the very end of the month the moon isn't even visible in the night sky!

When the sliver of a moon appears again, it is a sign that the new month has arrived. Even though these phases can easily be calculated by the members of the Sanhedrin, many of whom are astronomers in their spare time, Hashem wants all of the Bnei Yisroel to participate in the Mitzvah of Kiddush Ha-Chodesh. So at the beginning of the month it is a Mitzvah for Jews to look for that sliver in the sky and head over to Sanhedrin to bear witness to the sight of it. Of course, thousands of Jews could turn up as witnesses and the Sanhedrin will hear each and every one's testimony. When the Sanhedrin finishes listening to the testimony and determines that there are at least two independent, reliable eyewitnesses that the new moon occurred on a certain date, they declare Rosh Chodesh (first of the month) and fires were set on the hilltops to announce the new month to neighboring communities who, in turn, passed the message along.

Later on, torches were deemed not reliable, and messengers were sent out to proclaim the date.

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It took time for the messengers of the Sanhedrin in Eretz Yisroel to reach far away lands where Jews lived and let them know about Rosh Chodesh. It was therefore decreed that outside of Eretz Yisroel, in the Diaspora, festivals were to be observed for two days instead of one. This added second day was called "Yom Tov Sheini Shel Goliut - the second holiday of the Diaspora."

An exception was made in the case of Yom Kippur, which, because of the hardship of fasting, could not be prolonged. Rosh Hashana was also an exception in that it was observed for two days even in Eretz Yisroel, for Rosh Hashana was also the Rosh Chodesh, the first day of Tishrei. Even in Eretz Yisroel it could not always be ascertained on the preceding day whether the particular day was the first day of Tishrei or the last day of Elul.

Today, because we do not have a Sanhedrin, we have a fixed calendar, which has been figured out for us by Hillel II through mathematical and astronomical calculations. Despite this accuracy, the observance of "Yom Tov Sheini Shel Goliut," is still retained outside of Eretz Yisroel. Hillel's calculations are to be used until the coming of Mashiach (may he come speedily in our time).

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The cycle of the Jewish months provide a perfect picture of the colorful scene of Jewish life as expressed in our Yomim Tovim (holidays).

Spring Corresponding to: Yom Tov / Holiday:
NISAN (March-April) Pesach / Passover
IYAR (April-May) Lag B'Omer
SIVAN (May-June) Shavuot
TAMMUZ (June-July)  
AV (July-August)  
ELUL (Aug.- September)  
TISHREI (Sept.-October) Rosh Hashana, Yom Kippur, Sukkot
CHESHVAN (Oct. - November)  
KISLEV (Nov.- December) Chanukah
TEVET (Dec. - January)  
SHVAT (Jan. - February)  
ADAR (Feb. - March) Purim

(A Jewish leap year has two months of ADAR:
ADAR RISHON [the First Adar], and ADAR SHENI [the Second Adar].
Purim is in ADAR SHENI in Jewish leap years.
Jewish Leap years occur 7 times in a 19 year cycle. In the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the 19 year cycle (modulo 19), the extra month is inserted to keep the Holidays in sync with the seasons. (We'll explain soon enough).

Now, back to our original question:
Why do Jewish holidays fall on different dates every year?

We're finally going to tell you the reason.

THE HEBREW DATE OF JEWISH HOLIDAYS DOES NOT CHANGE FROM YEAR TO YEAR. Jewish holidays are celebrated each year on the VERY SAME DAY of the HEBREW month.
However, because the Jewish lunar calendar year is not the same length as the CIVIL (Gregorian) solar calendar year used by most of the western world, the Jewish holidays' CIVIL dates vary from year to year as the date shifts on the CIVIL calendar.

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BACKGROUND The Jewish calendar is based on the lunar year, and consists of 354 days, with each month beginning on the new moon. The Jewish month is measured by the time it takes for the moon to make a complete revolution around the earth. We speak of a month of this kind as a LUNAR month since it is based upon the time it takes the moon to make one whole revolution around the earth. (The month of the civil calendar is a SOLAR month, which is a twelfth part of the earth's revolution. The "solar" calendar is based on the relationship between the sun and the earth - 365.25 days per year.

The period of time it takes the moon to make a complete revolution around the earth is approximately twenty-nine and a half days.

Since we do not reckon a month in half days, because months are counted by days, not hours (Talmud Megillah 5a), it is necessary to add half a day to one month or subtract half a day from the next. We therefore count one month of twenty-nine days, and the following month of thirty days. A month of 29 days is followed by 1 day Rosh Chodesh. And a month of 30 days is followed by 2 days Rosh Chodesh. (More about this later).

As we said above, Rosh Chodesh, the first day of the month, begins when the first sliver of moon becomes visible after the dark period of the moon. The problem with strictly lunar calendars is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar loses about 11 days every year and a 13-month lunar calendar gains about 19 days every year. The months on such a calendar "drift" relative to the solar year. On a 12 month calendar, the month of NISAN, which is supposed to occur in the Spring, would occur 11 days earlier each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again.

To compensate for this drift, an extra month was occasionally added: a second month of ADAR. The month of NISAN would occur 11 days earlier for two or three years, and then would jump forward 29 or 30 days, balancing out the drift. This adjustment guarantees that Pesach will always occur in the spring.

As we mentioned above, Hillel II established the fixed calendar based on mathematical and astronomical calculations, in the fourth century, 4119(358/9 CE). This calendar, the one we still use, standardized the length of months and the addition of months over the course of a 19 year cycle, so that the lunar calendar realigns with the solar calendar. ADAR SHENI (II) is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The new year that began Monday, September 25, 1995 (Jewish calendar year 5756) was the 18th year of the cycle. Jewish calendar year 5758 (that began October 2, 1997) was the first year of the current cycle. Jewish calendar year 5760 (that began September 11, 1999) was the third year of the current cycle. Jewish calendar year 5763 (that began September 7, 2002) was the sixth year of the current cycle. All these are leap years with that extra ADAR II added to even the score.

The Months alternate between 29 and 30 days in length, with the actually moment of the new moon falling in between. The 30-day month is called "malei" (full) and the 29-day month is called "chaser" (deficient or short). In any given leap year, NISAN, SIVAN, AV, TISHREI, SHVAT and ADAR I are malei; IYAR, TAMMUZ, ELUL, TEVET, and ADAR II are chaser. CHESHVAN and KISLEV are sometimes malei and sometimes chaser. When a month is 30 days in length, the following month's Rosh Chodesh is celebrated for two days because the 30th day of the month past is counted as the first day of Rosh Chodesh and the first day of the subsequent month as the second day of Rosh Chodesh.

NISAN, SIVAN, AV, and TISHREI always begin with one day of Rosh Chodesh; IYAR, TAMMUZ, ELUL, CHESHVAN, ADAR I, and ADAR II always begin with two days Rosh Chodesh. KISLEV and TEVET vary between one and two days Rosh Chodesh.

In addition, Hillel saw to it that Yom Kippur should not fall adjacent to a Shabbat, because this would cause difficulties in coordinating the fast with the Shabbat, and Hoshanah Rabba should not fall on Shabbat because it would interfere with the holiday's observances. A day is added to the month of CHESHVAN or subtracted from the month of KISLEV of the previous year to prevent these things from happening.

The "first month" of the Jewish calendar is the month of NISAN, in the spring, when Pesach / Passover occurs. However, as we mentioned above, the Jewish New Year is in TISHREI, the seventh month, and that is when the year number is increased (5763, 64, etc.). The Jewish calendar has different starting points for different purposes. See our Tu B'Shvat page for more details.

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Month Length Civil Equivalent
NISAN 30 days March-April
IYAR 29 days April-May
SIVAN 30 days May-June
TAMMUZ 29 days June-July
AV 30 days July-August
ELUL 29 days August-September
TISHREI 30 days September-October
CHESHVAN 29 or 30 days October-November
KISLEV 30 or 29 days November-December
TEVET 29 days December-January
SHVAT 30 days January-February
ADAR 29 or 30 days February-March
ADAR II 29 days March-April

In leap years, ADAR has 30 days. In non-leap years, ADAR has 29 days.

The length of CHESHVAN and KISLEV are determined by complex calculations involving the time of day of the full moon of the following year's TISHREI and the day of the week that TISHREI would occur in the following year. Leave it to Hillel to figure it out. To make life easy for you, we have a Perpetual Jewish Calendar that will give you all the dates you can possibly want.

Note that the number of days between NISAN and TISHREI is always the same. Because of this, the time from the first major festival (Pesach / Passover in NISAN) to the last major festival (Sukkot in TISHREI) is always the same.

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The year number (5763, 5764 etc.) on the Jewish calendar represents the number of years since creation, as calculated by adding up the ages of people in the Bible back to the time of creation. So when we say that the year is 5763, that means 5763 years from the birth of Adam (you know, the first man in the Torah) on the 6th day (which may have been longer than the day as we know now, but that's a different story) of Creation. To give you a bit of perspective of where in time we are, Avrohom (Abraham) was born in the year 1948 of the Jewish calendar; Yetziat Mitzrayim (the Exodus from Egypt) was 2448, the modern state of Israel was established in 5708 (1948 CE).

The Jewish year is calculated by adding 3760 to the civil year, and conversely, the civil year is obtained by subtracting 3760 from the Jewish year.

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As we mentioned long ago, in the Torah, the months are never referred to by their proper names. They are always referred to as "the first month" ...(NISAN), "the seventh month"... (TISHREI)," and so on. The RAMBAN, in his explanation on the Torah (Shmot 12:2), quotes from the Talmud Yerushalmi (Rosh Hashana 1:2) that tells us where these names of the months came from: "The names of the months came up with us from Babylon." (During the time of Ezra, after the return from the Babylonian exile. The names are actually Babylonian month names, brought back to Israel by the returning exiles. Ed.).

The RAMBAN goes on to explain that at first, the Jewish people had no names for the months. The reason for the adoption of the names of the months when the exiles returned from Bavel to build the Second Beit Hamikdash stems from the fact that at first, the reckoning of the months was a memorial to Yetziat Mitzrayim (the exodus from Egypt). NISAN was the first month from (the anniversary of) our exodus, and so on. However, when the nation of Israel returned from exile in Bavel, the words found in Yirmiyahu (Jeremiah)(16:14-15) came true: "Behold, days are coming when it shall no more be said "As The L-rd lives, that brought up the children out of the land of Egypt, but: As The L-rd lives, that brought up and led the children of Israel from the land of the North, and from all the lands into which he had driven them."

After that time, we began to call the months by the names that they were called in the land of Bavel. Each time we mention the name of a month, we are thus reminded that we were in exile in Bavel, and from there Hashem redeemed us and brought us up to our land of Eretz Yisroel. Through the names of the months we remember our second redemption just as we had done with regard to the first one.

Yet, just because the names have changed, that does not mean that we no longer remember Yetziat Mitzrayim from the month names. In fact, when one looks at the times in the Torah where these Babylonian names are used, it is usually in the following way: "The first month, the month of NISAN." We have a reminder through the names of the months of both exiles that we, through the gracious hand of Hashem, were redeemed from.

To read the RAMBAN's own words, click here.

All twelve names appear as the main divisions of the Megillat Ta'anit (Scroll of Fasting). The twelve names and where they appeared first, are as follows:


The name appears only in Esther (3:7) and Nechemiah (Nehemiah) (2:1); elsewhere it is referred to as "the month of Aviv (Spring), Shmot (13:4, 23:15, 34:18), Devarim (16:1).


The Torah calls it Ziv (radiance) (Melachim (Kings) I, (6:1, 6:37), and it is referred to as IYAR in the Talmud, Rosh Hashana (1:3).


The name appears only in Esther (8:9).


It is mentioned once in reference to the Babylonian god, in Yechezkel (Ezek.), (8:14); In the Talmud it appears frequently as the name of the month.


The name first appears in Megillat Ta'anit (Scroll of Fasting), of the talmudic period.


Its name appears only once in Nechemiah (6:15).


In the Torah (Malachim 1/I Kings 8:2), Tishrei is referred to as "(B')Yerach HaAitanim - (In) the month of the mighty ones." It is given this name because the spiritually mighty Patriarchs were born in Tishrei; because the festivals of judgment, atonement, and joy with their many mitzvot occur in Tishrei (Talmud, Rosh Hashana 11a); or because the crops that are harvested in Tishrei give strength and health to the people (Radak).


The Torah calls Cheshvan "the month of the Bul" - Melachim I / Kings I (6:38) in reference to the beginning of the rainy season. The Torah tells us that the Great Flood ("Mabul") began on the 17th of this month. The waters came down in great force for a period of 40 days. Only a year later, on the 27th of this month, was Noach able to exit the Tayva (ark). Thus, the name, "Bul," may have been derived from the word "Mabul" - "flood." (See Abarbanel, Yirmiyahu / Jeremiah 3:3). It also appears in Talmudic literature and Josephus (Ant. 1,3,3).
Cheshvan is often referred to as MarCheshvan, i.e. with the prefix "Mar." The word "Mar" can be translated as a "drop." In this context, it refers to the fact that this month begins the rainy season in Eretz Yisroel. (See Pri Chadash, Even Ha'ezer 126:7).
There is also a popular misconception that the term "mar" means "bitter" due to its lack of holidays or because it is when Sara Imaynu (Sarah the Matriarch) died. Actually, the name of the month is M'racheshvan which means eighth month. It is referred to in the Mishnah and Talmud as Marcheshvan (some examples - Taanit 1:3,4; Pesachim 94b; and Rosh Hashana 7a, 11b). Whenever Rashi mentions this month, he calls it Marcheshvan.
Far from being a bitter month, the Bnei Yissaschar (2:56-57) relates a beautiful midrash that the dedication of the Third Beit Hamikdash will occur in Marcheshvan. May it come speedily in our days.


Mentioned in Zechariah (7:1) and Nechemiah (1:1).


Mentioned in Esther (2:16).


Mentioned in Zechariah (1:7).


The name appears in Ezra (6:15) and seven times in Esther, (3:7, 8:12, etc.)

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Hachodesh Hazeh Lochem Rosh Chodoshim Rishon Hu Lochem L'chodshay Hashana (12,2).

The Torah says. Hachodesh Hazeh Lochem Rosh Chodoshim - that the Bnei Yisroel should count this Chodesh as the first month and from this first one they will count all the months, the second, third, etc. until the end of the year - twelve months. The reason is that there should be a remembrance of the big Nes of Yetziat Mitzrayim, for when they say 'the first month' they mean the first month since Yetziat Mitzrayim. And so the 'second' and 'third' month they will always remember that Nes.

Therefore there are no names to the months in the Torah. The Torah only mentions them in the manner 'In the third month' (Shmot 19,1) or 'And it was in the second year, the second month…(Bamidbar 10,11) and 'In the seventh month, on the first day of the month etc. (Bamidbar 29,1). And so all of them.

Just as there are no names for the days of the week, they are counted as: 'the first day of the shabbat,' 'the second day of the shabbat,' this being so, so that there should be a remembrance of the shabbat day when the day of the week is mentioned, as I (Ramban) will explain (Bamidbar 20,8), so too we remember Yetziat Mitzrayim when we count 'the first month', the second… the third month'… of our redemption. Because we know that this count is not the count of months from the beginning of the year, as the beginning of the year starts with Tishrei, as it says, 'Vchag Ha-asif Tekufas Hashana' (Shmot 32, 22); 'Vchag Ha-asif B'tzays Hashana' (Shmot 33, 16). Therefore we must say that when we call the month of Nisan 'the first month' and the month of Tishrei 'the seventh month' the meaning is: First of redemption and seventh of redemption. That is the meaning of the phrase. 'Rishon Hu Lochem.' It is not the first month of the year, but it is the first to you - that you should call it 'the first' in memory of your redemption.

And the Rabbis have mentioned this inyan (topic). As they said (Yerushalmi Rosh Hashana, Chap 1, Halacha 2; also B'R 48, 9) 'The names of the months came up with us from Bavel' For in the beginning - until they left the Golut Bavel - there were no names of months in our vocabulary, and the reason for this is, because in the beginning the count of the months were a remembrance of Yetziat Mitzrayim, but when we went up from Bavel and the prophecy of the posuk** (verse) (Jirmiyahu 16, 14-15; also 23, 7-8) was fulfilled, we returned to call the months with the names they were called in the land of Bavel to remind us that there we stayed and from there we were redeemed by Hashem Yisborach. Those names, Nisan, Iyar.. (they) are Pharsi names, and they are mentioned only in the Books of the Prophets of Bavel, (Zecharia 1,7; et all; Ezra 6,15; Nechemia 1,1) and in Megillat Esther (2,16, et all). Even to this day the people of the lands of Poras and Madei, this is the way they call them (the months) Nisan and Tishrei and all of them… like we do.

** "And it will not be said anymore 'Chai Hashem' who took Bnei Yisroel up from the land of Mitzrayim, only 'Chai Hashem' who took and who brought Bnei Yisroel from the land of Tzafon."


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