I see three possible interpretations of this topic:
1) A literal interpretation---the end of a 24 hour day or the end of a workday or sunset on any given day
2) An idiom that seems to be used more in British English than in American English. The idiom means "when everything else has been taken into consideration". As in "At the end of the day it is her decision."
3) The end of a life or perhaps the end of an era.
I am not sure if any of these interpretations are correct for this topic---I am particularly uncertain about my third interpretation.
Or maybe there are other interpretations that I hadn't thought of?
Any suggestions or insights will be most welcome.