Maybe this is just because I'm still in school, I doubt it, but I've always thought of Monday-Friday and Saturday-Sunday as two different sections of the week. That's probably why one is called the week and the other the weekend. Friday for me always feels like the end of the week and the weekend is somewhat of a latent period until the beginning of a new week on Monday. Not that I don't do anything on the weekend, but it's a different routine.