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This is such a great question as they all are, because I sense that you have a feeling there is more than just the "rule" of tradition you have been given...

First of all, we know from the UB that this planet was not created in 7 days(nor were any other worlds)...in fact, it took millions of years through INTELLIGENT design by certain beings whose responsibility was to bring about worlds that could become inhabitable - where mortals such as us could live. If we, in our imperfection, need weeks and months to build huge buildings, then to perfectly create environments for creatures to live and breathe must take a MUCH longer time, yes? 

Now, Sunday or the Sabbath(Saturday in Jewish culture)...we need to go back to Adam and Eve:

74:4:4 (832.4) "It was near the dawn of their seventh day on earth that Adam and Eve heard the startling news of the proposal(to worship them as they seemed so heavenly!)of these well-meaning but misguided mortals; and then, even while the passenger birds were swiftly winging to bring them to the temple, the midwayers, being able to do such things, transported Adam and Eve to the Father’s temple. It was early on the morning of this seventh day and from the mount of their so recent reception that Adam held forth in explanation of the orders of divine sonship and made clear to these earth minds that only the Father and those whom he designates may be worshiped."

They had just spent the other 6 days after their arrival here being guided around the Garden, touring the various places set up there, studying the animals and plant life there, so many things to see! But they quickly corrected their admirers on to worshipping ONLY the Father and those who were his designates (like our Creator Father Brother)in the Garden Temple.

 74:4.5 "...Adam and Eve, moving apart from the throng, pointed to the Father’s temple and said: “Go you now to the material emblem of the Father’s invisible presence and bow down in worship of him who made us all and who keeps us living. And let this act be the sincere pledge that you never will again be tempted to worship anyone but God.” They all did as Adam directed. The Material Son and Daughter stood alone on the mount with bowed heads while the people prostrated themselves about the temple.

Look how they celebrated that day: 

74:4.6 (832.6) And this was the origin of the Sabbath-day tradition. Always in Eden the seventh day was devoted to the noontide assembly at the temple; long it was the custom to devote this day to self-culture. The forenoon was devoted to physical improvement, the noontime to spiritual worship, the afternoon to mind culture, while the evening was spent in social rejoicing. This was never the law in Eden, but it was the custom as long as the Adamic administration held sway on earth."

In the days when Jesus was here with his apostles:

138:6:2 (1542.7) "And they pursued this plan of relaxation for one day each week throughout the remainder of his material life. ... While Jesus, at this period of his earth life, did not actually require this day of rest, he conformed to this plan because he knew it was best for his human associates."

I think this day of rest was a Wednesday, mid-week.   

In any case, why would we not want to find rest during each day and pause to worship our Loving Father? Once a week, when we can join others of similar minds to celebrate his Fatherhood and our brotherhood.