The Talmud records a now-famous episode in which a prospective proselyte comes to Hillel the Elder and says he will convert on condition that the sage teach him “the whole Torah on one foot (al regel achas).”
Hillel responds by saying: “What is hateful in your eyes, do not do to your neighbor. The rest is commentary; go learn it.”
Reb Yisroel of Ruzhin offers this tantalizing, novel interpretation. He explains that the proselyte was really posing a question of much greater sophistication. He understood the cycle of the Shalosh Regalim— the three Pilgrim Festivals of Pesach, Shavuos, and Sukkos — and how they fit together so that the Jewish people could reexperience annually the physical and spiritual redemption of their ancestors.
What he did not understand was the regel echad— the One Festival of Shimini Atzeres, which is attached to Sukkos but not really part of it. His play on words, asking for an understanding “on one foot (regel),” was really an inquiry into the nature of the one Festival (regel) that remains apart from the other three.
Hillel answered him this way. Each of the Festivals celebrates a specific event and is defined by specific practices. Pesach commemorates the exodus from Egypt through the commandment to eat matzah; Shavuos commemorates the giving of the Torah at Sinai through the custom of staying up through the night learning; Sukkos commemorates the miracles through which the Almighty sustained the Jews in the desert by commanding us to move out of our homes into little huts.
Once all that is done, once we have reawakened and, we hope, revitalized our relationship with our Creator, one essential step remains: to revitalize our relationship with our fellow Jews. And so the Torah added an extra regel — festival — not commemorative of any event nor defined by any specific practice. By extending the festival season for an extra day, we have the opportunity to remind ourselves that, no matter how much we may strive to perfect our relationship with the Master of the Universe, we accomplish nothing unless we strive equally to perfect our relationship with our neighbors and fellows.
If we aren’t cautious, religious fervor and passion can become a source of dissension and division in the Jewish community. We are allowed our differences in how we adhere to Torah law; we are required to make distinctions between authentic Torah practice and those interpretations that have strayed from legitimate tradition. But in our conduct toward our fellow Jews, and in our passion for promoting unity within the Jewish community, there is not justification for not fighting against divisiveness with the same zeal we may have for attaching ourselves to the One G-d who charged us all, together, in His service.