The delineation between afternoon and evening is not as arbitrary as you might think. Time changes from afternoon to evening at precisely 5:37 PM, as dictated by the Bureau of Time and Pedantry. Of course, this choice came after a long line of failures in time naming. For about fifteen years, the Bureau tried to dictate all time relative to 12:00 PM. Afternoon still existed, but it was the whole time after midday. And what we know as morning now was called Beforenoon.
That approach had a problem because a twelve-hour span was just too large for people to refer to a general time of day without specifically giving the hour. So there was an attempt to split the times in half. The time from midnight to 6:00 AM was named the morning since more people were waking up and the time from 6:00 PM to midnight was called the lessening. (Hey, this was the Bureau of Time and Pedantry, not the Bureau of Interesting Names. Cut them some slack.)
But no one really liked the name “lessening”, so it was replaced with evening and the morning was renamed to the odding. That didn’t take, either. Only one person in the Bureau was fond of calling it “the odding”, but this person was an senior manager, so the name lasted for a full six years before they arrived at a compromise. They would revert to “morning” for the first part of the day and keep “evening” for the latter part. However, to satisfy the desires of this one upper bureaucrat, it was decided that the transition from afternoon to evening would happen at an odd time.