Well I'd like to think that Monday is the beginning. I do consider the week-end to be Fri, Sat, Sun A.K.A the Fun days as CGP said. Then I consider Mon to be the start of a new week which is why I hate Mondays but that's just me :p Not to mention I lose track of all of my day's order during the Summer & I always think it's a day ahead & it's just chaos!
If we're talking technicalities, then it's the beginning. But really, it should be the end, because people hate the beginning of the week because they have to return to work and such. So if Sunday is the beginning, how come no one says they hate Sundays? Because Monday is the real beginning.
I wish I could say it was the end of the week, but Cjenkel makes a very good point. In the beginning of doing something, we all have to gain the momentum and that's exactly why Mondays are so tiring for many people. I'd be contradicting myself if I said Sunday was the beginning, so I'll say it's the beginning but Monday is the beginning of the WORKweek.