The connection culminates in Jesus of Nazareth
Just after sundown on Preparation day, Jesus sat down with His disciples to break bread. That evening, He introduced the new covenant, which He was about to fulfill in part by the trial later that night followed by execution on the afternoon of Preparation day, then burial just before Passover’s start at sundown. He was resurrected the day after the seventh-day Sabbath, which would then be the eighth day and a new “first.” Several passages in the New Testament document this Matt 28:1, Mark 16:2, Mark 16:9, Luke 24:1, John 20:1.
After the resurrection, the Lord met His disciples on that same day, then again a week later on the eighth day,
John 20:19. The disciples met to break bread on the first day of the week Acts 20:7, 1 Cor 16:2, and Pentecost took place on the first day after Sabbath…the eighth day (reckoned according to fifty days or seven weeks after the waving of the firstfruits sheaf at the Feast of Unleavened bread). So the first day of the week or our Sunday became the day of gathering, and it continued, although it wasn’t mandated or called Sunday during that period. Sunday worship only became a sign of the Beast system when Constantine made it law and falsely giving it a stamp of approval by calling it “christian.”
In an excerpt from a letter to the Magnesians, Ignatius, a first century Christian described the transition from seventh day observance to eighth day or the Lord’s Day as it was called. He wrote of some "who were brought up in the ancient order of things," who "have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day."
Covenant (7) + Communion (1) = New Beginning (8)
Resurrection is a new beginning, and with that new beginning a new and everlasting covenant. As Christ stood in for us and became that covenant, we are of and in Him via the Eucharist (one body, one blood). When He said “this do in remembrance of me” Luke 22:19 and when Paul noted that “For as often as you eat this bread and drink this cup, you do show the Lord’s death till he come” 1 Cor 11:26, they were saying that each time you partake of this symbolic ritual, for as often as you do, you’re taking an OATH, which stipulates that you understand your actions, what the body and blood of Christ constitute and the dire consequences of your action if you do not recognize the blood as the new covenant and its bread as the body of Lord Jesus, the one born in the city of David… known as, “The house of bread” (Bethlehem).
Paul warned of taking the communion frivolously (1 Cor 11:27-34), and specifically he warned against taking communion with non-believers (1 Cor 10:16-21). Not incidentally, the word communion (koinonia) means fellowship but in this context it takes on the full definition: raising the right hand in a pledge (read: oath) of fellowship. Taking communion reaffirms the existence of the new covenant, which shall be fulfilled in its entirety at Judgment day.
Covenant (7) + Communion, body/blood of (1) = Resurrection and new beginnings (8). The eighth day man and eighth day symbols alluded to in scripture heralded a new and lasting covenant long awaited.
Becoming an “Eighth day man”
Col 2:14-17 "Blotting out the handwriting of ordinances that was against us, which was contrary to us, and took it out of the way, nailing it to his cross; (v.15) And having spoiled principalities and powers, he made a show of them openly, triumphing over them in it. (v.16) Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: (v.17) Which are a shadow of things to come; but the body is of Christ."
Verse 16 is crystal clear, no interpretation needed. More interesting: look at the last verse…”Which are a shadow of things to come, but the body is of Christ.” At first glance, shadow and body don’t seem to mesh, but in fact, they do. It’s the body that casts a shadow - rather than it being the shadow itself. He’s saying that the law from before with all its statutes, ordinances, feasts and yes, Sabbaths were projections on a wall, but not the substance of the Word. Just as a photograph is only a representation of you - not who you actually are. He is the law, the prophecies and the covenant. When we partake of Him, and obey Him, (not some letter, rather the heart of it), He being the sum of everything, then we are fulfilling the holy-days, Sabbaths and laws.
The problem people have following this premise is related to their tight grip on the flesh existence along with its concrete, non-spiritual understanding of what covenant is dead, and what is alive. It comes down to a lack of faith. He, the Son of Man and of God, born of woman made a covenant with God the Father. WE are not the covenant sealers. No one in Christ is getting circumcised, slaughtering animals or doing ritualistic cleansing. This covenant can’t be broken - ever.
By all appearances and contrary to their own words, many who strictly observe days, seasons and rituals (this covers the Jews as well as various Christian sects) are the ones in bondage to the law and are therefore under it, not because they still observe the old Sabbath, or because of diet, rather, because they fail to see what the new covenant is, what the old represented and what laws and ordinances were fulfilled at the Lord's death and resurrection. New covenant observers are neither bound by a carnal priesthood, nor by statutes and ordinances, nor by church-state mandated holidays. New covenant people consist of the priesthood of believers after Christ Jesus. And His priesthood is of the Order of Melchizedek, King of Salem (Hebrews 7).
No matter on what day you gather to worship, the prime consideration is becoming an eighth day man. The first step involves loving the truth, no matter how much you disagree with it, how much trouble it gets you in, or how it breaks and remolds you. No one can really know the Lord unless they’re willing to let go of everything for the truth, simply because He won’t divulge Himself to those looking for a purpose, a social life or a spiritual high. Once someone has that down, the rest is obvious, because truth-lovers race to read, study, interact and embrace the correction of the Holy Spirit. The essence of the eighth day man is to allow your Creator to transform you from that of the mere sixth day creation, to that of a resurrected person, who has passed into the Sabbath (being Christ Himself) and who is ready to take the eighth step up to the inner sanctuary.