When does the "week" begin?   stance: my week begins as shown on calendars, sunday:
As tempting as the "week debate" pun is (and it is!)....
Isn't it bad enough that the year supposedly starts in the middle of winter, so that each year gets TWO winters? Why would we want to split up our weekend so that it's only one day long on each end? A work-week is five days, then two days of play; lather, rinse, and repeat. Or take the "week" off and go skiing, but that's neither here nor there.
I prefer calendars that start with Sunday 'cause it's prettier (symmetry!), but window-dressing aside, the week starts on Monday.