Quote:
001:003 And God said, Let there be light: and there was light.
001:004 And God saw the light, that it was good: and God divided the
light from the darkness.
001:005 And God called the light Day, and the darkness he called
Night. And the evening and the morning were the first day.
That is how it reads.
The light/energy of the Big bang, as you suggest, cannot and was not separated from the darkness to create day and night.
The only thing that separates day from night is the spinning of the earth in relation to our sun. But in genesis the sun doesn`t exist yet.
-I would add that this was an understandible mistake for the time it was written.