According to Nehemiah 7-9, after Nehemiah and company rebuild the temple and the wall, they came to the seventh month of the year. On the first day of the 7th month (Day of Trumpets) they celebrated by reading the entirety of the book of the law. The people mourned and were then cheered up by those who had read it saying that they should rejoice, and celebrate with feasts, as they now understood that which God wanted for them.
They celebrated and, upon realizing that they should celebrate Feast of Tabernacles, went out to gather branches and put sukkot together on their roofs and in their yards; it states that the people had not celebrated in this way since the time of Joshua, son of Nun. At the conclusion of the Feast of Tabernacles the people wore sackcloth and ashes and joined in the temple to repent of their sins.
Links:
Day of Trumpets Feast of Tabernacles Repentance
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