On the day following the seven-day Feast of Tabernacles—the eighth day—the Israelites were to observe a final Holy Day in the annual cycle God gave (Leviticus 23:36-39).
Though a separate festival, the Eighth Day is connected to the Feast of Tabernacles or In-gathering because it further celebrates the future spiritual harvest of mankind, portraying events that will immediately follow the millennial reign of Christ and His resurrected and transformed saints.
The 1,000-year period of peace and prosperity for the entire global human family serves as a stepping stone to a vast increase in the late harvest of human lives. Revelation 20:5 says that "the rest of the dead” will live again when the 1,000 years are finished. This includes all people who lived but were not converted to God’s way from Adam to Christ’s second coming—which could number in the scores of billions.
They will be raised to physical life and at last given the opportunity for salvation during a period of judgment (compare Ezekiel 37:1-14; Matthew 11:20-24; 12:41-42; 1 Peter 2:12; Revelation 20:11-12). This is sometimes called the Great White Throne Judgment, as Revelation 20:11-12 describes the risen dead standing before such a throne.
The White Throne Judgment is not an instant sentencing but a period of evaluation and corrective measures spanning the new lifetime of those resurrected—paralleling the lifetime judgment of God’s saints today, whom He wants to save, not lose (see 1 Peter 4:17; 1 Corinthians 11:31-32).
Sadly, not everyone will choose salvation. Those who ultimately refuse to repent will be destroyed in a lake of fire (Revelation 20:14-15; 21:8). But the vast majority of mankind will be saved—transformed into immortal spirit like those in the first resurrection.
Indeed, the whole creation will be transformed. The great beauty, peace and prosperity on earth during Christ’s millennial reign will continue through the White Throne Judgment period. And after this, the present earth and sky will give way to, as mentioned earlier, a new heaven and new earth—at which time God the Father as well as Jesus Christ will dwell with glorified humanity forever (Revelation 21-22).