As I read it, a day means one period of sunlight and one period without. I suppose that is the same as 24 hours by our standards.
Gen 1:1 says "In the beginning", it does not say "At first". Thus we know right away that this is a story of the events, but not necessarily in the order in which they appear, nor do they completely detail all of His works.
So what should be interpreted is that "In the beginning" really means is that "In the beginning of God's creating", but not necessarily in the order.
Additionally, the Hebrew word for "heavens" is a compound word of "Fire" and "Water" -- which also indicates that fire and water pre-existed Heaven and earth.
The "24-hour day" that is observable on most of the earth is the result of the Earth's rotation making the Sun alternately visible and invislbe to a ground-based observer. The Sun did not exist, however, until the Fourth Day. Therefore, something else must have created the light and darkness for the first three "days"; I see no reason to believe that those "days" were of the same duration as one rotation of our planet.