In Leviticus 23:36 God adds another day to the seven day celebration of Tabernacles. Here it is termed the eighth day. God pronounces it to be a Holy time of gathering. As was noted with the observance of Trumpets, there is not a great deal said about the observance of this Feast, so we look then to what it foretells. For those that can believe, we see and hear Christ Jesus speaking as to the prophesied time of the fulfillment of the eighth day Feast of God (John 7:37-39). According to (Revelation 20:7-15) after the thousand year reign of Christ here on this earth has expired, a series of events will take place. Among these is the dethroning of Satan for eternity, and also we see the resurrection of the rest of the dead. After being resurrected, those of this resurrection must be judged (Revelation 20:12). We proclaim by the word of God that judgement is now, in the present upon those that believe in Christ (1 Peter 4:17). Christ said to His disciples, "To you it is given to know the mysteries of the kingdom of heaven, but to them it has not been given" (Matthew 13:11).
The Apostle Paul writes about Christ's Gospel being hidden (2 Corinthians 4:3-4). Our God declares He is not a respecter of person (Luke 20:21). So when will He grant those to whom it is hidden to hear and see that they too are judged according to those things written in the books? (Revelation 20:12) We believe that in God's plan of salvation, He has allotted the Last Great Day as that period of time for their judging, for then the books will be opened to them, even as they are now being opened to those that can believe.
Again it is our prayer that God will both grant you to understand and to begin observing His Feast days.