Sunday - Beginning of the week or end of the week?

Current Song in Head: Uncle Mutton, Don and Drew

Ok, I'm having a debate with my little princess. Is Sunday the start of the week or the end of the week? My stand is that the Sunday is the end of the week cause it is called the weekends right (including Saturday)? But my little princess thinks that Sunday is the beginning of the week because most of the calenders start with Sunday, including her precious handphone.

I've done some research and found that Christians usually assume the beginning of the week to be Sunday but yet the International Standard Organisaion has decided to use Monday as the start of the week. So which is which?

I'm sure that people have different opinions and I respect that. But I for one would be sticking to believing that Monday is the start of the week, otherwise, I might have to start going to work on Sundays. And don't even get me started on how the Arab countries take the weekends to be Thursdays and Fridays...