An eight day feast of ingathering.
THE LAST OF THREE COMPULSORY PILGRIMAGE FEASTS:
Travel to the Sanctuary in Jerusalem required of all men, to present themselves before the Lord (Exo 23:14,17, Exo 34:22, Deut 16:16).
The first day was the 15th day of the 7th month (Ethanim / Tishri)
Exo 34:22, Lev 23:34-36, 39-43, Num 29:12
A convocation / sabbath day. No servile work done.
Travel to the Sanctuary in Jerusalem required of all men (Exo 23:14,17, Exo 34:22, Deut 16:16).
Fruit Harvest - Firstfruits presented to the Lord (Exo 23:19, 14:1-5)
For seven days all Israel moved out of their homes and lived in temporary shelters called "Sukkah" as a reminder of their wanderings in the desert for forty years. The branches cut from palm (Rev. 7:9), willow and other trees were to be waved in celebration to the Lord during the first seven days of the feast (Lev 23:40).
This final feast of the year is a celebration of ingathering at the end of the harvest (Exo 23:16) and is a time of rejoicing and fellowship. It symbolizes the gathering or harvest of God's people, who leave earth for the week long marriage supper of the Lamb, to be celebrated at the Father's house in heaven after the second coming of Jesus (Rev. 19:7-9). This begins the millennium, where the saints will dwell temporarilly until the earth is made new, after the judgment of the wicked (Rev. 20).
The 7th day of the Feast of Tabernacles, Festival of Willows (Hoshana Rabbah)
21st day of the seventh month.
Considered by Jewish custom to be the final day of judgment. A ritual of beating willow branches on the ground is practiced, which is thought to symbolize the casting away of sin.

Reference: The Biblical Feast Days. Retrieved on Feb. 29, 2016. http://biblelight.net/feasts.html