"Then God said, "Let there be something to divide the water in two." So God made the air and placed some of the water above the air and some below it. God named the air "sky". Evening passed, and morning came. This was the second day."
The second day of Creation and already God was creating the ability for life to come. The separation from the water, some below the air and some above it, was the basis for the water cycle, something which the planet needed if it was ever going to be able to support life.
Personally I am of the opinion that the '7 days of creation' are metaphorical days, which in reality refer to large periods of history much longer, possibly as long as millenia. As God didn't actually create man until much later in the period it is possible that they refer to Days as God would view them. I once read a trilogy of books, by Terry Pratchett which was comprised of 'Truckers', 'Diggers' and 'Wings'. Those books were about people who were only 10 inches tall, or somewhere in that region, they were similar to the Borrowers, but for them a single one of our days lasted in the region of a week. I may be getting the details incorrect, it's been a while since i read them, but it gives you an idea. Anyway the reasoning for this is that they were smaller so for them time passed faster, they still noticed our night and day, but they also noticed more minor fluctuations in the light which we take no notice of. My reason for bringing that up is because I think it's a valid comparison between ourselves and God. We are so much smaller, so much less significant than him, that what would be a day for him would last a phenomenal amount of time longer for us, and as he was the only one around it would be he who would tell us, and he would tell us in terms of his own time span.
Using that as my scenario I would place this 'Day' of creation at roughly the couple of millenia where Carbon started to come out of the rocks and form into CO2 which would in turn support my theory both Scientifically and Philosophically.