Well in school we are taught that Sunday is the first day of the week when we are learning he order/name of the days (kindergarten/preschool usually)
I personally am not religious, so I don't look at it from that religious perspective, although I am highly educated in that area
I don't know. I usually think of Friday and Saturday as the weekend because those are the 2 nights that you get to actually go out and have fun, whereas Sunday nights are usually no different than school nights.
OH and calendars usually start the weeks on Sunday, end on Saturday.