I notice both the sides (far left and far right) of the issue saw the people before them all
debating this and they decided what they believed and then joined one side or the other. Iraniaus said "it is a prophecy" that is he was speaking of the six days of creation and the rest in Genesis. His explanation was about the mark of the beast(or the number),he said "six hundred because Noah was six hundred when he entered the ark and sixty and six because the Image Nebuchadnezzar required them to bow down to was sixty cubits tall and six cubits wide",,a while later he made the comment that this surmised all the worlds apostasy from the deluge throughout all mankind's history and that in "six thousand years it would be concluded".,,,
I wondered,,"why on earth did Iranieus tie the seven days in genesis,the destruction(deluge) the ark Noah,the image in Danial the forth commandment all together with the end and the mark of the beast",(other things also as you say it is repeated throughout the bible). In (Issiah 46;10) it says that in the beginning he told us the end thereof,so "beginning/days of creation and the rest/sabbath" is also telling us the (end) thereof,so as well as the story of creation is telling us the creation(genesis) it also is telling us the end(judgement day) and the day of rest(millennial kingdom).
I am glad to see that you also notice the prophecy found in the same(maybe different details). I think that to most I must seem like a slippery fish when we all discuss the sabbath (Saturday verses Sunday) because it seems I support both,that is Saturday I think cannot be removed because it is a message with a meaning God is trying to convey to us,but the same the first day(the Lords day) is also a message with a meaning,they both go together to form a complete sentence being conveyed by God. That is as well as the six days are conveying the length of time mankind will labor,and the seventh the rest, the day of the Lord is conveying the day after these sets of prophecy.