I believe the Days of creation are God’s days not earthly days. They are therefore analogous to the human work week. I also believe that they overlap with each other. Each day when God speaks initiates the creative fulfillments. After God commands a parenthetical comment follows stating that it indeed takes place. It doesn’t say when. Evidence that the days overlap come from at least two areas. One of them is in Psalms 104. Keep in mind there is no evening and morning on God’s Seventh Day and as Hebrews 4 tells us we are still in God’s Seventh Day of rest. Yet the psalmist describes the natural death and rebirth of animals in psalms 104 by saying God is still creating:
You hide Your face, they are dismayed; You take away their spirit, they expire and return to their dust. You send forth Your Spirit, they are created; And You renew the face of the ground. - Psalms 104:29-30
Here we have the outworking of day 5 and 6 overlapping God’s seventh day of rest. For God rested long ago and is still resting even though the psalmist speaks of Him creating animals during his lifetime after they die.
Another piece of evidence that the days overlap is in Genesis 2. Genesis 1 says “Let the land produce animals.” It then says “Let us make male and female”. Genesis 2 has God making man and then making animals because man needs a helper in “the land”. No suitable helper was found so God then makes woman. The order isn’t contradictory. God’s creative fiats overlap one another.