There is a progression in our avodas Hashem that occurs during the month of Tishrei. This reflects itself in the role that simcha plays on each of the yomim tovim. On Rosh Hashana and Yom Kippur it is questionable as to whether there is a formal mitzvah of simcha. Although there was a tradition in some communities to recite the phrase "moadim l'simcha chagim u'zmanim l'sasson" on the Yomim Noraim as we do on the Shalosh Regalim, the practice today is to delete these words. According to the view of Tosafos that the mitzvah of simchas yom tov can only be fulfilled by eating special korbanos offered for this purpose, this form of simcha is clearly lacking on Rosh Hashana and Yom Kippur when such korbanos are not offered. The Rambam describes the Yomim Noraim as days of not-excessive joy, indicating that there is some degree of simcha present albeit not on par with Pesach, Shavuos, and Sukkos.
As we transition to Sukkos, not only is there definitely a mitzvah of simcha, but in addition Sukkos is referred to as "zman simchasenu." Simcha is not tangential, but rather is an integral component of the Yom Tov. In contrast to Rosh Hashana and Yom Kippur, which the Rambam describes as days of not-excessive joy, the days of Sukkos are days during which the level of joy is complete. The Rambam explains that the unique celebration of simchas beis ha'shoeva in the Beis Hamikdash during Sukkos was an expression of the halachic requirement of excessive joy that applies specifically to Sukkos. The excessive joy that is absent on the Yomim Noraim becomes present on Sukkos because of the transition that occurs between these two periods of time. The Rambam observes that the Yomim Noraim are lacking in this intense joy because the intensity of the judgment that is occurring prevents such joy; at a time of Divine Judgment it is impossible to be in a state of total joy. It is the realization that we emerged victorious from the time of judgment that is the source of the great joy of Sukkos. As Chazal describe, when the books of life and death are open on the Yomim Noraim we cannot say Hallel. Once we are inscribed in the Book of Life our joy has no bounds.
Since a renewal of life is the source of our great joy on Sukkos, as we transition to Shmini Atzeres and Simchas Torah our joy becomes even more elevated. We dance in response to the realization that Torah is our very life; the gift of life that has been granted to us can now be infused with talmud Torah and shemiras ha'mitzvos and thus we truly celebrate life. The month of Tishrei is truly a celebration of being inscribed and sealed in the book of life. Just as we dance on Simchas Torah holding dearly the ultimate book of life (i.e. the Torah), so may we merit to live our lives in great joy inscribed and sealed in the book of life.
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