And God called the light Day, and the darkness he called Night. (Genesis 1:5a ASV)
Light = Day
Darkness = Night
Here we see God naming the separated light and darkness “Day” and “Night”, respectively, similar to his naming the firmament “Heavens” on the 2nd Day, and the dry land “Earth” and the gathered waters “Seas” on the 3rd Day. He is providing us with a Glossary of Terms in this first chapter to help us understand their use throughout the rest of Scripture.
Unfortunately, so many take what we’ve come to understand in life (outside of Scripture) and have imposed those understandings onto these terms, specifically the term “Day”, as they are used in Scripture and have misunderstood what Scripture is really saying to us.
Day as it Relates to the Sun
For example, many have the notion that the sun defines the Day, however in the Genesis 1 account, there is no “sun” at this point in Creation…there is only “light”.
First comes the light, which is called “Day” and the darkness, which is called “Night” (verses 3-5). Then come the “Heavens” (verses 6-8), the “Earth” and the “Seas” (verses 9-10). After these are established, then the sun is put in the heavens and is used to rule the Day (verses 14-18).
It is the light that defines the Day, and then the sun rules it.
A 24-Hour Day
Another false notion is that the Day can also be inclusive of the Night. We are taught from early childhood that a 24-hour period of time, which includes both “Day” and “Night”, is also called “Day”. Sadly, this has brought so much confusion.
According to Genesis 1 the Day by its very nature is the light. It cannot include the darkness or else it would cease to be what the Creator called Day.
This understanding works throughout the whole Bible, from Genesis 1:5 to Revelation 21:25, it’s final mention. I encourage all to read through the Bible again with this understanding to see what I mean.
What we consider a 24-hour period of time would be equivalent to “day and night” or “night and day” in Scripture. And what Scripture considers a “day” is simply “the light”.
To better understand what the Creator is teaching us, we need to use the Glossary of Terms provided by Him from the very beginning.
Darkness is called Night. Light is called Day.
EDITED 1/11/17 based on new-to-me information.