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Understanding the Jewish Calendar: The Math & the Psychology

The Two Phases of Jewish History in Responding to Crisis

2 hr

Class Summary:

This class, presented on Sunday Parshas Bo, 2 Shevat 5777, January 29, 5777, at Ohr Chaim, Monsey, NY, explores the structure of the Jewish calendar.

Given the inherent challenges of a lunar based calendar (including the fact that it needs to be synchronized with the solar seasons), why did the Torah choose set up the calendar and base Jewish life around the moon? Also, what is the significance of the fact that for the first half of Jewish history we established our Rosh Chodesh based on live witnesses who observed the moon; yet, for its second half, the system was altered to follow a pre-set calendar. And if the calendar is a fair way of navigating time, why did the Torah demand that the optimal way of establishing the first day of the month is through the tedious and complex process of witnesses traveling to the court and being cross examined there? Why not always use the calendar?

Third, why is the mitzvah of sanctifying the new moon told to Moses as a prelude to the redemption of the Jews from Egyptian bondage? What connection is there between the moon’s renewal and the Egyptian Exodus? And why right now? It is strange that with so much going on, G-d decides at this precise moment to discuss the Jewish calendar! The end is near; liberation is about to happen. Why are we now concerned with the calendar?

Fifth, why was this the first mitzvah given to the Jewish people, days before they were to become a free nation? 

In a deeply moving address in 1959, the Lubavitcher Rebbe offered a majestic and heart stirring explanation.


Please leave your comment below!

  • Anonymous -6 years ago

    Thanking you for this shiur from last year, some fascinating insights from The Rebbe. I shared the contents with my children Friday night.

     With regards to the waxing and waning of the moon vis-à-vis Jewish history, i wanted to share with you a fascinating Maharal. Taken from chiddushei aggodos Shevuos 9a but with regards to the gemoro in Chulin 60b – the conversation between the moon and Hashem.

     At the end it says, that the korban we bring on rosh choidesh (this week’s sedra) is a kaporo for Hashem and only through this korban was the moon pacified.

     In brief the Maharal explains, that kaporo for Hashem means wiping the slate clean which is what a korban does. The very smallness of the moon is the catalyst for the Jews to bring the korban of Rosh Choidesh and thereby get close to Hashem. The moon complained l’sheim shomayim, for the honour of Hashem. And this is how it was finally pacified. Your very smallness, ie, on Rosh Choidseh tells the yidden – bring a korban and thereby they get close to me.

     I thought this tied in beautifully with the Rebbe’s words that at exactly the point of despair and lowness is the exact moment of beginning to come close and renewal.

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  • Anonymous -6 years ago

    Moshe could not figure out so Hashem had to show him

    was Machatsis Hashekel one of them?

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  • RB

    Rivki Berkovits-Rabinowitz -7 years ago

    It seems that the rebbes explanation על פי נסתר for the before and after of קידוש החודש has very much resemblance to the מחלוקת between ריש לקיש and ר'יוחנן about pain in last week's פרשה. Apparently it shows further why we פסקין like ריש לקיש.

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  • MZ

    Mendel Zilberberg -7 years ago

    As I listened to the shiur I couldn't help thinking that your
    analysis of the two intervals of Rosh Chodesh - re'eah and the formal calendar
    are possibly similar to the Lubavitcher Rebbe's life and a new interval where there is a
    generation or generations who can no longer "see" but progress and prosper
    because they "know"

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  • I

    Isaac -7 years ago

    I did not understand something. Even outside of the Holy Land, during Second Temple times and afterward, Jews needed to pray a special Musaf prayer on Rosh Chodesh. So why did they not need to celebrate two days of Rosh Chodesh since they did not know when to recite this Musaf prayer?

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    • RYJ

      Rabbi YY Jacobson -7 years ago

      Good question. The answer may be this: Since davening musaf on Rosh Chodesh is only a rabbinic obligation, the Rabbis can say that you may do only one day rosh chodesh and daven musaf at that day even if it is a doubt if it is the right day. But with the other holidays, there are biblical mitzvos that have to be done, so the Jews living far away had to celebrate two days of Yom Tov so as not to violate biblical obligations and prohibitions. For example, the Rabbis can't say, do not celebrate 8 days of Pesach, because then Jews living in these places may be eating chametz on pesach or doing melacha on yom tov, or will not fulfill the mitzvah of eating matzah, etc. Same with other holidays.

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Rabbi YY Jacobson

  • January 29, 2017
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  • 2 Sh'vat 5777
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Leilu Nishmat Reb Eliyahu Tzion ben Reb Chananya Niasoff ז"ל 
And in the merit of our partner in Torah Yigal Yisroel ben Sofia שיחיו 

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