Genesis 2:2-3 And on the seventh day God finished his work that he had done, and he rested on the seventh day from all his work that he had done. So God blessed the seventh day and made it holy, because on it God rested from all his work that he had done in creation.

From God's actions in the creation story so far, we can assume that he is infinitely powerful. Why would an all-powerful creator choose to rest? Why would he need to separate creation into days at all?