GENESIS 1:8
 

And God called the firmament Heaven. So the evening and the morning were the second day. [Genesis 1:8 NKJV]
 

And God called the firmament Heavens. And there was evening and there was morning, a second day. [Genesis 1:8 AMPC]

Bible Teacher:

This "Heaven" came into being when the evening and the morning were the second day. But before then, there were the Heavens that came into being when it was in the Beginning and when there was neither evening nor morning. Thus:
 

• ★ This Heaven(s) of the second day was a physical Heaven (Sky) which was first called Firmament (Expanse) before it became called Heaven. But
   
• ★ Those Heaven(s) created in the beginning were spiritual Heavens (spiritual Realms).
   

As there's physical Heaven and physical earth, there's is also spiritual Heaven and spiritual Earth created in the beginning or eternity before time began.
 

Commentaries:

While God was giving the Firmament a new name, "Heaven," Time and Numbering were renaming that Day with a new first name or new number, "Second Day" or "Day Two." Evening and Morning continued to event even in the midst of the separated waters where the Firmament of Heaven was. Evidently, Time and Numbering were not just counting over the face of the waters above the Heaven, but also counted in same frequency within and beneath the waters below as well as in the Heaven or the Firmament. 
 

Bible Teacher:

Who said there's no time in heaven? Who said there's no Numbering in heaven? God called the Firmament, "Heaven;" and yet Time numbered the Second Day. Therefore, Heaven is not Timeless, but part of a Timeline.
 

The firmament of Heaven is Timed, and the stars and bodies therein are numbered with the age of Times. Signs of the Times also spring up from the Heaven and its host therein. There's Time for everything in heaven and under heaven. Therefore, this Heaven and this Earth shall pass away when there's no more time for them all to continue being.
 

It was in the second day this heaven was made and expanded to receive the creations of God, and the heaven received the switching timer between the evening and the morning. Time or regulation of day and night or of light and darkness, was the first to fill that heaven after God. And when that heaven shall be no more, time will be changed.
 

Commentaries:

When God was calling it "Firmament in the midst of the waters" God was bringing it into being and using it divide the ancient floods. But when God called it "Heaven" God turned it into a sanctuary, tabernacle or dwelling place, for Celestial thing. Thus, this heaven is not just a Firmament that stretches or divides the waters from the waters, but a sanctuary and domain that houses and upholds powers, glories and hosts of heaven.
 

REVIEW OF THE EVENTS BY THE SECOND DAY
 

In the second day, was the firmament of heaven established and empowered to reign over the midst of the waters and to declare the works and glory of God forever.
 

★ In the first day, God made the light and established the light as "Time" to reign and count the measures of opportunities or chances allotted to each thing in creation.
 

★ In the second day, God made the firmament and established the firmament as "Heaven" to reign and declare the works and glory of God in God's creation.

But the second day was the only day in the first six days we were not told "God saw it was good," since it was already Heaven. Since it was heaven, it was good because God it called heaven.