Why should Christians celebrate the biblical festival called the eighth day or the Last Great Day?

The seventh and last Holy Day in God's cycle of harvest festivals has great meaning for Christians today.

The final step in God's plan of salvation for all mankind is portrayed in a feast day that immediately follows the Feast of Tabernacles (Leviticus 23:39Also in the fifteenth day of the seventh month, when ye have gathered in the fruit of the land, ye shall keep a feast unto the LORD seven days: on the first day shall be a sabbath, and on the eighth day shall be a sabbath.
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Called the eighth day or the Last Great Day, this festival pictures the Great White Throne Judgment of humankind described in RevelationThe disclosure of God's Word and plan to mankind. In the Bible this refers to making obscure things clear; bringing hidden matters to light; causing especially called individuals to see, hear, perceive, know and understand the things of God; the unveiling of biblical mysteries (Romans 16:25). 20:11-13 [11] And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them.
[12] And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.
[13] And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.

See All.... During this time all people who have died not knowing God's plan for them will be resurrected to life to be given an opportunity to have their minds opened and respond to God's call.

All the dead who have ever lived—including the queen of Sheba, inhabitants of ancient Nineveh and the people of Christ's own time—will all be resurrected together (Matthew 12:41-42 [41] The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here.
[42] The queen of the south shall rise up in the judgment with this generation, and shall condemn it: for she came from the uttermost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here.

See All...). That resurrection of multiple billions of people back to physical life is detailed in Ezekiel 37:1-14 [1] The hand of the LORD was upon me, and carried me out in the spirit of the LORD, and set me down in the midst of the valley which was full of bones,
[2] And caused me to pass by them round about: and, behold, there were very many in the open valley; and, lo, they were very dry.
[3] And he said unto me, Son of man, can these bones live? And I answered, O Lord GOD, thou knowest.
[4] Again he said unto me, Prophesy upon these bones, and say unto them, O ye dry bones, hear the word of the LORD.
[5] Thus saith the Lord GOD unto these bones; Behold, I will cause breath to enter into you, and ye shall live:
[6] And I will lay sinews upon you, and will bring up flesh upon you, and cover you with skin, and put breath in you, and ye shall live; and ye shall know that I am the LORD.
[7] So I prophesied as I was commanded: and as I prophesied, there was a noise, and behold a shaking, and the bones came together, bone to his bone.
[8] And when I beheld, lo, the sinews and the flesh came up upon them, and the skin covered them above: but there was no breath in them.
[9] Then said he unto me, Prophesy unto the wind, prophesy, son of man, and say to the wind, Thus saith the Lord GOD; Come from the four winds, O breath, and breathe upon these slain, that they may live.
[10] So I prophesied as he commanded me, and the breath came into them, and they lived, and stood up upon their feet, an exceeding great army.
[11] Then he said unto me, Son of man, these bones are the whole house of Israel: behold, they say, Our bones are dried, and our hope is lost: we are cut off for our parts.
[12] Therefore prophesy and say unto them, Thus saith the Lord GOD; Behold, O my people, I will open your graves, and cause you to come up out of your graves, and bring you into the land of Israel.
[13] And ye shall know that I am the LORD, when I have opened your graves, O my people, and brought you up out of your graves,
[14] And shall put my spirit in you, and ye shall live, and I shall place you in your own land: then shall ye know that I the LORD have spoken it, and performed it, saith the LORD.

See All.... These are the "rest of the dead" spoken of in RevelationThe disclosure of God's Word and plan to mankind. In the Bible this refers to making obscure things clear; bringing hidden matters to light; causing especially called individuals to see, hear, perceive, know and understand the things of God; the unveiling of biblical mysteries (Romans 16:25). 20:5But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.
See All.... Billions of human beings will have the opportunity to repent of their past sins and receive the gift of everlasting life.

For those who choose not to surrender to God and His way of life (and would hence live a miserable existence), a merciful and permanent end to their lives will be met in the Lake of Fire, which will occur at the conclusion of this final judgment period (RevelationThe disclosure of God's Word and plan to mankind. In the Bible this refers to making obscure things clear; bringing hidden matters to light; causing especially called individuals to see, hear, perceive, know and understand the things of God; the unveiling of biblical mysteries (Romans 16:25). 20:11-15 [11] And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them.
[12] And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.
[13] And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.
[14] And death and hell were cast into the lake of fire. This is the second death.
[15] And whosoever was not found written in the book of life was cast into the lake of fire.

See All...). Even though not everyone will choose God's way of life for eternity, our Creator wants "all men to be saved" (1 Timothy 2:4Who will have all men to be saved, and to come unto the knowledge of the truth.
See All...) and is "not willing that any should perish but that all should come to repentance" (2 Peter 3:9The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
See All...). Through God's wonderful plan everyone will have an opportunity to know His truth, repent and receive salvation.