Jews call the Last Great Day, the seventh and final of God's annual Feasts, Shemini 'Azeret (1906 Jewish Encyclopedia). This phrase means "eighth day assembly" as God commanded his people to meet on it for a holy assembly (Leviticus 23:36). The Bible simply calls it the "eighth day" (Leviticus 23:36, 39, 2Chronicles 7:8 - 10) or "the great day of the feast" (John 7:37) as it immediately follows the Feast of Tabernacles.
The Last Great Day, when no work is allowed (Numbers 29:35), occurs each year on Tishri 22 on the Hebrew calendar (September - October).
Surprisingly, very little is written about the Last Great Day in Scripture. In the Old Testament, it was a time when, like the Feast of Tabernacles, people gathered to worship the Eternal and offer thanksgiving for his abundant blessings.
Old Testament addition
At Jerusalem's temple, a ritual known as the water pouring (libation) ceremony took place during the Feast of Tabernacles and on the Last Great Day. This practice was added even though it was not commanded in God's law. It is unknown who started the tradition and when it was first adopted. What is known is that it was extant during Alexander Jannaeus' service as High Priest that ran from 103 to 76 B.C.
The Jews believed the libation ceremony primarily represented the pouring out of God's Holy Spirit (Isaiah 12:3). Its lesser meaning was that it was a visual representation of prayers that asked God to grant the land abundant rain (Temple at the Time of Christ by Edersheim, Chapter 14).
It is in the New Testament, however, that we discover the full symbolism of the eighth and "great" day. Jesus' words offered during the water pouring (libation) ceremony, coupled with the book of Revelation, reveal its awesome truth.
New Testament revealing
Jesus is recorded as observing the Last Great Day in 29 A.D. (John 7:37), less than six months before he is crucified. The Lord was among the morning crowd at Jerusalem's temple to witness the water pouring (libation) ceremony. When the water was being offered at the inner court's altar (or possibly after it was completed) he, with a loud voice so all could hear, cried the following.
"If anyone thirsts, let him come to Me and drink. The one who believes in Me, as the scripture has said (see Isaiah 44:3 - 4, 58:11), out of his belly shall flow rivers of living water" (John 7:37 - 38, HBFV).
The Feast of Tabernacles symbolizes Christ's millennial reign on the earth. He, along with the righteous who were part of the first resurrection, will "harvest" countless humans during this time for God's kingdom. The Last Great Day follows up and fulfills this theme by solving the problem of all those who were not called to salvation from Adam to Jesus' return.
A full chance
God, in what is called the Great White Throne Judgment (or second resurrection), will resurrect all those humans who lived up to the time of Jesus' return who never received a full opportunity to be saved (Revelation 20:11 - 13). This includes all those who never heard the gospel or heard parts of it but were never given a heart to comprehend it fully. It also includes the countless millions of aborted babies who were murdered for the sake of convenience!
This huge mass of people will have their minds opened to the truth and given a chance to repent. Their change of heart, as Jesus alluded to in his Last Great Day offer (John 7:37 - 38), will cause them to spiritually thirst for what is right and true. Anyone willing to "drink in" God ways and obey him will ultimately be given the gift of eternal life! This is the meaning of this joyous period!