For fun if we did not have days of the week then you would never have the weekend off.lol. Now since the sun and the moon take turns coming up( as the earth turns on it's axis and does it's orbit around the sun) and going down then I would say it is common sense that we(being humans with given knowledge ) would have determined that this would separate the days from the nights and since we separate the days and nights then we(humans) came up with naming the different days. Then we came up with the names of the seasons and the months and so on.
