Perhaps the most confusing and contested passage in all of Barnabas is 15.3-9. In his refutation of the Jewish practice of keeping the Sabbath, Ps.-Barnabas writes this with reference to the six days of creation (Gen 1):
He speaks of the sabbath at the beginning of the creation: “And God made the works of his hands in six days, and finished on the seventh day, and rested on it, and sanctified it.” Observe, children, what “he finished in six days” means. It means this: that in six thousand years the Lord will bring everything to an end, for with him a day signifies a thousand years. And he himself bears me witness when he says, “Behold, the day of the Lord will be as a thousand years.” Therefore, children, in six days–that is, in six thousand years–everything will be brought to an end. “And he rested on the seventh day.” This means: when his son comes, he will destroy the time of the lawless one and will judge the ungodly and will change the sun and the moon and the stars, and then he will truly rest on the seventh day. […]
Finally, he says to them: “I cannot bear your new moons and sabbaths.” You see what he means: it is not the present sabbaths that are acceptable to me, but the one that I have made; on that sabbath, after I have set everything at rest, I will create the beginning of another world. This is why we spend the eighth day in celebration, the day on which Jesus both arose from the dead and, after appearing again, ascended into heaven.
Got all that? Me neither. What’s going on here is this: many Jewish and early Christian writers divided human history into seven millennia, with the seventh day as the Sabbath rest. For a chiliastic writer like Irenaeus (that is, he believed in a literal earthly millennium before the creation of the new heaven and new earth), the seventh day was the millennial period. But for Ps.-Barnabas, both the seventh and the eighth days appear to be Sabbaths. Are these days identical? Or are they sequential? Or is Ps.-Barnabas merely a clumsy redactor who has mixed up his sources? All of these views are well represented in the literature.
Pretty much all of the discussion surrounding the eschatology of Barnabas has focused exclusively on this passage. Following the insight of Jonathan A. Draper (“Barnabas and the Riddle of the Didache Revisited,” JSNT 58: 89-113) that the “epistolary frame” of Barnabas (that is, the “letter-like” chapters 1 and 21) can function as in interpretive key to the whole, I’m analyzing the eschatology of these chapters to see if it can shed light on the problem of chapter 15. It remains to be seen whether or not I’ll be able to produce a convincing argument, but at the least any chance to work in the Apostolic Fathers (and particularly Barnabas) is a welcome opportunity.
Kyle R. Hughes 
Looking forward to your insights!
On a seven day week, the first and the eighth are essentially the same day, with this one difference–the eighth day represents the re-creation or new creation. If you were an Old Covenant Israelite, reading the divine prescriptions concerning the observance of the Old Covenant ceremonies, you would be compelled to ask the question, “Why, if the Sabbath Day, from creation to the giving of the Law on Sinai, was the seventh day of the week, is there explicit reference to eighth day Sabbaths attached to the festivals?” For instance, in Leviticus 23:36-39–at the institution of the Feast of Tabernacles, the LORD commanded Israel:
For seven days you shall offer an offering made by fire to the Lord. On the eighth day you shall have a holy convocation, and you shall offer an offering made by fire to the Lord. It is a sacred assembly, and you shall do no customary work on it…on the first day there shall be a sabbath-rest, and on the eighth day a sabbath-rest.
Note especially how the Lord prescribed a first and eighth day Sabbath during the Feast of Tabernacles. This alone ties together the point about them having identity in their theological significance.
In the resurrection of Jesus on the first day (i.e. the eighth day), the glory of God’s presence is made manifest to His people. Jesus brings about the new creation through His incarnation, death and resurrection and so fulfills the Feast of Tabernacles.
The Sabbath day prefigured the need for eternal and new creation rest. Picking up on the time element of the Sabbath, Iain D. Campbell, in his very helpful work, On the First Day of the Week, made the following observation on the significance of the eighth day in the prophecy of Ezekiel’s eschatology Temple:
Things will be different in the future, but the Sabbath principle will remain. The blessings of communion with God, of which the Sabbath speaks so eloquently, will be enjoyed in new measure by the people of God. Iain Duguid brings out the importance of this, when he comments on the ‘lack of timelessness’ so often found in eschatological visions in the Bible. He goes on to say that ‘in Ezekiel’s reordering of the festival calendar, time itself is brought under the discipline of the new age’, and he goes on to apply this to Christian worship today. And although he does not explicitly speak of the Sabbath factor in Christian worship, that is surely one of the main lines along which Ezekiel’s vision takes us: to the realization that just as Jesus is our sacrifice and Prince, and just as we are a spiritual temple in him, so he has given us a new sacred ‘time’, a new Sabbath, a Sabbath of the eighth day (cf. 43:27), our Lord’s Day Sabbath.3
While these truths certainly have implication for the theological shift from the bloody sign of circumcision to the unbloody sign of baptism, and from the seventh day to the first day (eighth day) for the Sabbath, they teach us much about the fulfillment of all things in the death and resurrection of our Lord Jesus. The storyline of the Scriptures is the story of the new creation through the death and resurrection of Christ. By His death on the cross, Jesus brought about the new creation. Peter T. O’Brien makes the following helpful observation about the meaning of the phrase “the circumcision of Christ” in Colossians 2:11 when he suggests, “It is better to regard the statement as denoting the circumcision that Christ underwent, that is, His crucifixion, of which His literal circumcision was at best a token by way of anticipation (cf. Bruce, 234).4 His death was a bloody circumcision that brought about the circumcision (made without hands) in the hearts of His people. When he was cut off in bloody judgment under the wrath of God, He was providing all that was necessary for the cutting away of the guilt, corruption and power of our sin. By His resurrection, Jesus ushered in the new creation, by both raising His people up to newness of life now as well as by securing our bodily resurrection and the New Heavens and New Earth wherein righteousness will dwell at the consummation. The “eighth day” is pregnant with ceremonial significance in redemptive history. As with all the types and shadows ordained by God, it was invested with theological significance to serve the redemptive historical purposes of God
Jesus was NOT resurrected on the 1st/8th day. He was resurrected on God’s 7th day. The Gospels are clear on this.
This was not a mistake. Israel up into the New Testament kept a lunar solar calendar. Note there were no clocks or computers and they happen to have a calendar that was perfect that would be used for their feast days. The New Moon day is the first day of every month. 7 days from that day is eight days. It is because of the New Moon day. See Psalms 81.3 and 104:19 and Ezekiel 46:1-3. Hopefully you do know that the current calendar the world follows is pagan called the Gregorian and Israel didn’t follow this back then.