I suspect most people are aware that the biblical day begins at sunset or early evening sometime if one can’t see the sun actually set. However, there are people who would disagree with this biblical interpretation promoting sunrise rather than sunset as the starting point. Others, such as myself, are more than willing to set them straight since fixing the other guy is more entertaining than fixing oneself.
Some don’t care when the day starts so long as they can get to work on time. Today we can mechanically compromise and let some calculation or satellite pick out a dividing point between yesterday and today such as the stroke of midnight when the Cinderellas of the world best make sure they’re back home.
However, the fourth commandment expects us to keep the Sabbath (aka Sunday) holy. How are we going to do that if we don’t know when He wants the Sabbath to start and end? It is after all His commandment, not ours, to get some rest. Others think their acceptance of Yeshua (Jesus) allows them do what they want. They might be right. It might not be a salvation issue, but we may still be making a mess of our lives by not doing what He wants while we have the opportunity.
Psychologically, the reason we are tempted to think the day starts at sunrise is that is about the time we get out of bed. That is when we start doing stuff. If what we do is all that matters perhaps only the time from sunrise, or earlier dawn, to sunset, or later dusk, is all that matters. Forget about the night or turn on the light.
In Genesis 1 the first day starts in darkness. Elohim creates the heavens and the earth, but the earth is formless and void and darkness is over the deep and the Spirit of Elohim moves over the surface of the waters. There is a lot that Elohim is doing before He says, “Let there be light.”
By analogy with that first day our days need God’s handiwork on us after we stop working at sunset and before we get our turn to hopefully not make a mess of things at sunrise. That might be one reason to see why the day should start at sunset. We stop working (eventually) and let God offer dreams, insight, calls to repentance, and warnings with suggestions that we really do owe Him praise and thanks.
The sunset-start day puts what God does first. It makes sure what we do later during daylight hours is subordinate to what He wants, not what we want, not even what we think is possible for us to do. That’s the main reason I favor having the day begin at sunset, the time we stop working and acknowledge He is in control to begin the new day as the old one ends.