Sunday has always (FSVO) been the start of the week - it's not an American thing at all (indeed I've found Europeans get more confused when you don't start a week on Sunday). The Sabbath - the seventh day of the week referred to in Genesis - was Saturday, and Jews still celebrate it as such - Christians moved a lot of the way it's treated to Sunday.
"Weekend" is just a catchy name; "weekboundary" would be much more cumbersome.
Although "end", when used for time, usually refers to the chronologically later boundary of a period, there are other senses of the word in which an end can also be a start: a line has two ends, and we say "from end to end", for example.
Thus, I don't think it's necessarily contradictory to speak of an end of a week that also comes at the start - and that's without even thinking about the inherently cyclical nature of the week.
"Weekend" is just a catchy name; "weekboundary" would be much more cumbersome.