Do you have an idea why would one day be more important than the other for some people?
I have a hypothesis regarding the commandment to rest every seven days.
First, though, think of how odd it is to have an omnipotent god that requires six days to create reality, and how odd it is that they would then say that this god needed to rest for a day. There's usually a reason for something like that, just as we need an explanation for the story of a global flood rather than the type of flood humanity had experience with, and making it about a fickle, cruel god that thought that it could fix its engineering mistake using the same breeding stock - a not very flattering depiction of this god.
Here's my guess regarding the advent of the Sabbath.
Once, before the advent of organized, centralized religion, I presume that it was considered sinful for any able-bodied person capable of working to not work simply because he wanted a day off. The flocks needed attention every day. If it was planting or harvesting season, there were no days when planting or harvesting didn't occur until the job was done.
Fast forward to the advent of organized religion with a priesthood and centralized gathering places for religious purposes. The priesthood needs the people to come to it and to the temple, which requires that people put down their plowshares, travel to the synagogue for services, and to bring tithes to support the priests.
This round trip and meeting would likely take most of a day for many of the people served by any given synagogue, and require that the farmer, smith, or shepherd take a day off work to travel to the priests - once considered sinful sloth. A new ethic was born. You will take one day away from your labors each week
In fact, it was commanded under penalty of stoning, which commandment made the top ten list. The sin then was working on this day. And God's day of rest serves as the role model, and why it is sinful to not also take a day off each week.
Notice also the choice of the week, an artificial construct with no astronomical correlate like the day, month, or year, which are inspired by celestial motions and cycles. How often shall these people be instructed to take a day off and bring tithes to the priests? A month was too long, and a day too short, so, the week was invented - the work week to be precise - and the first weekend.
Now the story make sense. Now we have a plausible reason for there being six days of labor for this god followed by one of rest.
Incidentally, to those who say that the days of Genesis are not meant to be literal days, the idea of a seventh day of rest being 24 hours pretty much settles that for me.
Regarding the flood story, my guess is that it exists because people found shells and marine fossils on mountaintops, couldn't conceive of the mountains rising from sea floors and hence postulated a global flood, and then needed to invent a reason why a good god would drown the world and end up with what he intended to remedy - a sinful humanity.
This story depicts the god as intellectually and morally challenged, which we wouldn't expect these people to do without a good reason. They needed to account for ocean life on mountaintops.
Now that story makes sense as well.