The Mishna and Gemarah define the seventh day of Sukkos as the day designated to the mitzvah of Aravah; the Zohar sees it as the final day of judgement and atonement. But why this day? How did Hoshana Raba become a day of atonement?

It was the Rogatchover Gaon who in a letter suggested a dazzlingly novel idea. It was the day that the entire Jewish nation brought an offering of atonement for a major sin they all committed by mistake: eating and drinking on Yom Kippur. 

At the peak of the celebration for the dedication of the First Beis Hamikdash, the Supreme Court of the Jewish people made a terrible miscalculation. They needed atonement. And that gave us the holiday of Hoshna Rabah. Not only did this major collective failure not destroy the joy; ultimately it transformed the people of Israel into Baalei Teshuvah, and allowed them to reach even greater spiritual and emotional heights.