“God called the expanse heaven. And there was evening and there was morning, a second day.” (Genesis 1:8)
Important parts of God’s creation are the names given. The sense of the word “expanse” is a support. The expanse supports the waters above it. For this reason, the word is also translated “firmament”. Again, there is a great catastrophe later in Genesis where we will see the impact of these supported waters (Genesis 7:11).
The word for “heaven” is used with the sense of sky. The same word is also used for what we call space, as well as the place of God’s dwelling etc. From the context we learn how the word is being used here, sky. This is the observational language previously mentioned.
To stress the point, again for this post, the second day has an evening and a morning – one day, no more or less.