When God said, let light be, the sacrificial lamb was slain from the foundation of the world. That's why it went dark at the end of the first day, the second occurrence of darkness that we have to account for called night.
Let Light Be
Darkness Did Not Comprehend It
Why did it get dark at the end of the 1st day? It wasn't absence of light, it was obstruction of light by the corruption which resulted from the appearance of light. It needed a new name: night.