Ending or Beginning?

For some, this may be a stupid question but does Sunday represent the beginning of the new week or the ending of the old week?

I guess my Bible training leads me to believe that it is the ending of the week because God made the world in 6 days and rested on the 7th day, giving me the impression that Sunday is the end of the week...

Yet, most if not all calendars have Sunday as the beginning of the week with Saturday ending the week...  of course that could just be to separate the two "S" days from each other.

WHY EVEN WORRY ABOUT THIS SHIT, YOU MIGHT BE THINKING RIGHT NOW?
And, that is a valid question...
But...  for me, it is still of concern because some of life's details influence the way I think and how the week begins is important, especially since the work week begins with Monday.

Monday begins...

Sunday ends...