How could people not understand a day? The only way to not understand a day would be to not have gone through a 24 hour period of light and dark, and see there is a cycle here that repeats itself, and call it a 'day'. I dare say people have a vivid understanding of a day long before they knew how to speak, let alone teach others.
To say that a day was used as a placeholder for seven instances of time implys that there were seven phases of creation. Take the first instance, when he made light, and the heavens, and the earth, to be the big bang, and creation therefor after following a natural path to get us to a toxic yellow sky earth, and the following days to be the history of earth, with plants, and then animals forming, followed by humans showing up, and then on the last day god resting, and having no more infulance over the world.
Well this makes sence in a metaphorical veiw! We would still be in the seventh 'day', or the time after the initial creation, were as god no longer models the world, and instead we, as humans, are masters of the sky, land and sea.
Just one tiny problem. All storys in the bible happen after this event, and most involve god interacting with the world. So we can take the first story of genisis as being metaphoricaly true, but every story from then on is false, or else god isn't resting on the seventh day (The time after the first 6 phases of creation, what were in now) at all!