So the evening and the morning were the fourth day.
Genesis 1:19
Surrounding context
1:18 and to rule over the day and over the night, and to divide the light from the darkness. And God saw that it was good.
1:19 So the evening and the morning were the fourth day.
1:20 Then God said, "Let the waters abound with an abundance of living creatures, and let birds fly above the earth across the face of the firmament of the heavens."