I'm so sick and tired of this argument, but let me explain.
For a lot of people, they see the weekend as the CLOSING of the week. My mom sees Saturday and Sunday as the weekend because she doesn't have to work those days, and she's free to do what she wants. Thus, it's now the end of her week, and Monday starts a new week for her, since she has to go to work.
For me, I see Friday and Saturday as my weekends, since I have my calendar set up to start on Sunday, and when I was in school, Friday night and Saturday were the times I'd hang out with friends most often, as Sunday was spent getting ready for school the next day. Now I work on Saturdays in the morning, but I see Sunday as the beginning just because, well I just do.
It's completely subjective.