And God saw the light, that it was good, and God made a distinction between light and darkness. And God called the light day, and the darkness he called night. — Genesis 1:4-5
The light is said to be good because it is from the Lord, who is goodness itself.
The darkness is whatever looked like light to us before our new conception and birth, because we saw evil as good and falsity as truth; but it is actually darkness—our lingering sense of self-sufficiency.
Absolutely everything that is the Lord’s is compared to the day, because it belongs to the light, and everything that is our own is compared to the night, because it belongs to the darkness. The Word draws this comparison in quite a few places.
from Secrets of Heaven, Volime 1, Section 21