For the Jewish culture, the 8th day is tied to circumcision -- the day in which little boys joined the people of God by submitting to the mark of the covenant. And while these little Jewish boys certainly didn't have any choice in the matter (they were only 8 days old!), they did have a choice with whether or not they remained the people of God. Over and over in the OT God's prophets compare the chasing after false gods with sexual immorality (with some quite lurid passages). Faithfulness to the mark of the covenant (and it's association with the male sexual organ), then, is connected with faithfulness to God.
So, the whole notion of God's people and God's covenant is symbolized by the 8th day.
For us on the other side of the cross, the number 8 is about New Creation, but first let's go back to the beginning. In Genesis 1, God creates the world in 6 days and rests on the 7th. At the end of each of the first 6 days, there is a common refrain: "And there was evening and morning. . .", but this refrain does not appear for the 7th day. And the reason for this is simple: because that day has not yet ended for God. He is still resting from His work of creation. He is still active, obviously, just not creating. But there is coming a day in which He will take that creative work up again -- the 1st day of a New Creation week. According to God's calendar, the 8th day.
This is why Jesus rose from the dead on the 1st day of the week -- the firstfruits of those who will be resurrected bodily (he's the only One who's enjoyed this thus far). This is also why Pentecost occured on the 1st day of the week -- the Holy Spirit endwelling God's people as the Deposit of something greater to come (Eph 1:13-14; interesting point: the word "deposit" here is the word they would use for the engagement ring, the "promise" of something better for a future Bride).
In the OT, the 8th day represents covenant. In the NT, it represents New Creation.
It's interesting then, how Paul puts these two concepts together in Col 2:11-12.
In him you were also circumcised, in the putting off of the sinful nature, not with a circumcision done by the hands of men but with the circumcision done by Christ, having been buried with him in baptism and raised with him through your faith in the power of God, who raised him from the dead.
In baptism, both understandings of the 8th day come together. It is the mark of the covenant between us and God, and it is the sign and symbol of New Creation -- a preview of our future resurrection.
And so, just as the Jews needed to be faithful to the mark of their covenant (and not go lusting after pagan gods), so Christians are called to be faithful to theirs. What does this look like? Let's keep reading in Colossians:
Since, then, you have been raised with Christ, set your hearts on things above, where Christ is seated at the right hand of God. Set your minds on things above, not on earthly things. (3:1-2)
But now you must rid yourselves of all such things as these: anger, rage, malice, slander, and filthy language from your lips. Do not lie to each other, since you have taken off your old self with its practices and have put on the new self, which is being renewed in knowledge in the image of its Creator. (3:8-10)
Therefore, as God's chosen people, holy and dearly loved, clothe yourselves with compassion, kindness, humility, gentleness and patience. Bear with each other and forgive whatever grievances you may have against one another. Forgive as the Lord forgave you. And over all these virtues put on love, which binds them all together in perfect unity.
Let the peace of Christ rule in your hearts, since as members of one body you were called to peace. And be thankful. Let the word of Christ dwell in you richly as you teach and admonish one another with all wisdom, and as you sing psalms, hymns and spiritual songs with gratitude in your hearts to God. And whatever you do, whether in word or deed, do it all in the name of the Lord Jesus, giving thanks to God the Father through him.
(3:12-17)
It's interesting how Paul talks here about community life: don't lie to or slander each other, but clothe yourselves with compassion and love, and let peace rule in your hearts. Whether or not we are faithful to the covenant we sealed at our baptism has a lot to do with how we treat one another.
Let's treat each other as we really are: brothers and sisters baptized into God's family and awaiting the 8th day together.