Yom Gadol (The Last Great Day) is a new covenant feast and part of the Doctrine of Messiah (Heb 6: 1-2). See the excellent article on this website from 10/20/19 by Bob Bondurant regarding Yom Gadol, and his article from 8/25/18 regarding the New Covenant Feasts of Yahweh.
Prior to the Last Great Day, the adversary will be put away for a thousand years during the Millennial reign of our Creator (Rev 20:1-3). During this millennial reign, children will live to an advanced age and old people will not die. (Isaiah 65:20) Since the adversary is put away (no evil spirit will be influencing people's minds), all flesh will know our Creator since ONLY the Holy Spirit will be entering their thoughts.
After the thousand years are completed, the adversary will be "loosed a little season" (Rev 20:3). Satan shall go out and deceive the nations (Rev 20:7-9). He will prepare to battle and encompass the camp of the Saints in Jerusalem, similar to the Battle of Armageddon prior to the millennium. The Battle of Gog and Magog ends poorly for the adversary and likely the millions of humans that knew our Creator perfectly but were unable to overcome the adversary when he was let out of his prison after the millennium (Rev 20:8-10).
Paul confirms in 1st Corinthians, chapter 6, verse 3 that the Saints will judge the fallen angels in the last great day. Jude confirms in chapter 1:6 that the angels that have fallen are reserved in darkness, that is to say Satan's kingdom, which is this life or the darkness below.
The first hundred years of the last great day is a day of salvation for all flesh and with their perfect knowledge of our Creator gained during the millennium they all will have their chance to overcome the adversary which allows the growth of Messiah and gives them a chance to be in the family as part of the innumerable multitude. So today is a day of salvation for the 144,000 that overcome prior to the seventh trumpet. The first hundred years of the last great day is a day of salvation for all others who are not part of the 144,000 (Isa 49:8). All are given a fair chance to be in the family!
Revelation, chapter 20:12-13, shows that the books are opened (people's deeds) in comparison to another book that is opened, most likely the Bible. Remember every bit of creation including the fallen angels get the free gift of life (Rev 20:14 death is cast into the lake of fire), however, to be in the family requires being called and chosen and the work or deeds is overcoming the adversary. Remember he loves all his creation unconditionally; he just doesn't like our deeds and wrong thoughts.
Remember the Hebrew word olam, or “forever”, is just a period of time as Jonah was in the belly of the great fish forever (olam), but it was just 3 days! Our Creator is not a terrorist. He is not going to punish people eternally even if he allows our physical bodies or the fallen angels spiritual bodies to be burned. They will likely be dead bodies within a few microseconds and their mind or spirit or pneuma will return to the Father asleep (Ecc 12:7, Psa 90:10) until they are given a spiritual body in the new heavens and earth.
Truly, the Last Great Day is graduation day for all creation and is the great plan of all time to be completed by the Son (Alpha & Omega) and the kingdom handed over to the Father from the Son. Pray to the Father in the name of his Son for wisdom and read all of Revelation for blessings, as I don't want to add to or take away from any of our great Creator’s words.
If this inspires you, please consider praying to Father Yahweh in the name of his resurrected Son, YahwehShua, for wisdom and read the other articles on this website starting with the earliest and going forward.
May Yahweh bless your understanding.