Genesis 1:20

"And God said, 'Let the water teem with living creatures, and let birds fly above the earth across the vault of the sky.'"  Genesis 1:20

In the Interlinear Bible it translates creatures as "living beings" and birds as "flying creatures."  So here God says to let the waters be full of living beings and flying beings fly in the expanse over the land.  So every creature that flies or swims is what he is about to create.  He's already created the vegetation, so these creatures will already have food and shelter provided for them.  And they'll already have night and day and the stars and the sun and moon.

At Pbcc.org they have the creation week divided into 2 sets.  The first being the forming of creation and the 2nd being the filling of creation.  The filling of creation actually started with Day 3 with God creating the sun, moon, and stars for the light he created on the very 1st day.  So the 4th day is filling the sky and seas that God created on the 2nd day.  An interesting thought, that 1st God creates a place for the creature and then he creates the creature.  He's doing the same for us now.  He's creating a home for us in heaven, before he comes again to turn us into a new creature.  (Not that I would really call the sun, moon, or stars creatures, but there is a lot we don't know about them, so maybe they could be...)