That’s William Blake’s “The Ancient of Days” to bring together two concepts of Days and Creation. Quora.com included the question, What bible book is most controversial? I said Job, not Genesis. Unless you don’t understand metaphor. Then I was asked, what is the metaphor of “Day” in the creation story? My answer:
If you mean, what is meant by the term “day”, as if to challenge the riduculous notion that 24 hours is long enough for the development of the cosmos, or the seas, or all vegetation, then certainly, we could prefer “day” in the sense of “day of judgement” or “in the days of Abraham”, which usage is figurative rather than metaphorical, and refers to any period characterized by its activity.
More interestingly, In Gen 1,14 “Day” is created by separating light from night, and this happens on “Day four”, ignoring the fact that three “days” have passed already! If we knew Hebrew, perhaps we’d look for two different words for day. Or regardless derive from this that the figurative “Days” one to three cannot be astronomical days.
If on the other hand you mean, what is the metaphor of the seven progressive days (or periods, or sequenced activities), that’s what modern rational philosophy would call the “Teleological” nature of creation – but in poetical story form. As with understanding icons, stories must be accepted as an ancient and rich medium that do not merely pre-date but actually surpass our modern favourite medium, rational language, in their ability to contain and convey ideas across boundaries of generations or literacy. At any historic stage of scientific “knowledge”, the story telling of this sequence of “Days” could convey the deliberate order and purpose that the author finds empirically evident in the structure of creation.
The use of the term, “Day” also conveys the authority and power of the designer through the immediacy with which this ordered intention is instantiated. However, if we accept the story’s premise of prior intent and creative power, then the issue of the length of the “day” becomes irrelevant! The unimaginable power to create from nothing but intent is no less awesome done slowly than quickly, and the benign and purposeful nature of the design is so much more significant in the story’s meaning.