And God said, Let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness. And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
On the first day, God commanded light to shine out of darkness and established the day and night cycle. This is a picture of the original separation of light from darkness and the conversion of the sinner, which has been brought from darkness into the light.