Jesus Christ is the Light of the first Day. He came forth from the invisible Spirit of God to overcome the darkness or death which contaminated the air (heaven) dust (earth) and water which God created in the beginning. Genesis 1:1-2
Jesus speaks of this first Day just before His crucifixion, in the Garden of Gethsemane.
Jhn 17:5 And now, O Father, glorify thou Me with thine own self with the glory which I had with Thee before the world was.
Since there was only ONE Day before the world was made, Jesus is revealing that He is the Light of the first Day. This also explains how there could be plants, which grew the 3rd Day, ((Genesis 1:12) BEFORE the Sun, which was made the 4th Day.