Because God it the source of light - I said that above. If he wills a day to "exist" before creating the sun and stars then it can be, and there are other sources of light besides our sun, you did know that right ? And this is really an issue of semantics anyway.  I am claiming the Bible as the source of truth, I am not attempting to prove anything to you as it is fruitless to do so, you are a hater of God, that is clear who chooses to believer her own fairy tales.