[cgriswald]

I disagree with the sibling poster. Your first paragraph is correct.

The week starts on Sunday. The weekend ends on Sunday. It only makes sense if you consider two adjoining endpoints of previous weeks to be one "weekend", which I've never known anyone not to do without thinking about it.

Someone talking at work might mean something different, but I don't usually hear "next week" to mean Monday-Sunday in general conversation and we'd probably clarify for Sunday anyway.

If someone says "The week of the 15th" and Sunday is the 15th, they mean the seven days from 15-21 not the seven days of 9-15.

"Next week, maybe Sunday" means the next calendar Sunday as in Sunday-Saturday, not the Sunday after that as in Monday-Sunday.

Anyone who says otherwise is selling calendars with a Monday start of the week. :)