Hi @Mike_Gantt,
Thanks for hanging out here and for your honest inquiry. I realize I’m quite late to the party, but would like to share some of my own thoughts. I have been reading along and understand that most has already been covered in one way or another, but I decided to join in anyways. I’ll just address the topics as expressed in your OP.
Six days
Now, what helps me is the following. Imagine being a spiritual leader to the people of Israel, who have just escaped from years of slavery under the reign of the pharaoh of Egypt. Imagine it is your task to explain to them who JHWH is and what He did, and why He is different from the pagan gods.
To explain His role as the Creator, you would have to explain how His creative activity differs from that of the pagan gods. The pagan creation stories usually involved gods who fought with each other in a cosmic battle, after which the winner would become the “king” of creation. In contrast, in Genesis 1 we read of JHWH who does not have to lift even a proverbial finger. He just speaks and everything simply happens as He commands. Speech is invoked to explain to finite human beings that God has absolute authority over Creation. For them, the power of the word of a king was equal to his authority (remember Jesus calming the sea by His word? 
). This part also makes clear that all the objects in Creation (e.g., sun, stars, seas) do not have any authority of their own (i.e., are not gods), but simply are creations of God.
Also, this Creation narrative would have to be placed in a context that peasants would understand. What would be a useful motive to employ? I cannot think of anything better than a working week, something that should have been already familiar to the Jews even before the Sabbath was officially installed. This is where the 6+1 days come into the story. This framework helped establish a system of worship for the Jews, in which they reminisce and celebrate God’s creative activity every week. God’s rest is the end of the week, which I will discuss now.
Seventh day
Well as others probably have pointed out, the number seven itself carried special significance for the Jews. It signified completion to them, as does the end of the working week. So yes, there is something “historical” about this seventh day, in the sense that it signifies the completion of God’s establishment of Creation. Again, this is a good way of bringing across God’s creative activity to an Ancient Near-Eastern audience. There are indications in Hebrews 4 that this same rest of God is continuing even today:
9 There remains, then, a Sabbath-rest for the people of God; 10 for anyone who enters God’s rest also rests from their works, just as God did from his. 11 Let us, therefore, make every effort to enter that rest, so that no one will perish by following their example of disobedience.
God’s resting “day” is everlasting and we are taken up in His rest as believers. This is an interesting point from the Bible, indicating that the interpretation of those days was not meant to be taken nearly as rigidly as we (with our modern Western mindsets) might be prone to do.
Summary
In summary, I believe there is a powerful two-way relationship between the 6+1 structure of the working week and the 6+1 structure of Genesis 1. I view this as a Spirit-inspired decision by the biblical author. On the one hand, the 6+1 structure of Genesis 1 (with its connection to the working week) helps the common person to understand God’s creative activity with an illustration from daily life. On the other hand, the 6+1 structure of the working week (with its connection to Genesis 1) gave the Jews a practical framework for worship and reverence of God in their daily lives, especially with the focus on the Sabbath as a resting day, celebrating God’s Creation. Given this two-way relationship, I don’t think that the physical duration/chronology of God’s creative acts was the focus of the biblical author or his audience. Instead, the author made the God-inspired choice to connect and ground (or even “incarnate”?) God’s creative acts in something accessible to those who were receiving these Scriptures for the first time. This is comparable to the way the prophets spoke of the New Creation using earthly images, because the actual reality of the New Creation lies beyond our human imagination.
Peace,
Casper