Sunday. I have no clever reason to believe that, it’s just what I was taught. The first time I was ever aware of being proven wrong was in Kindergarten when I was on the wrong side of the “how many days are their in a week?” question. Five, of course, then the weekend.
Sunday.
(Does anyone have a computer file with the Falk calendar on it? It is a manufacturing/scheduling calendar that follows the format YYWWD (year, week of the year, day of the week). On it, Monday through Saturday are days 1 through 6, however, I have worked with different operations that numbered Sunday either as Day 7 or as Day 0. And let me tell you that coordinating calendars when two divisions of the same company number Sunday differently is a chore.)
In my corner of the retail world, our fiscal weeks are Sunday through Saturday. All goals are set this way.
Last year my boss thought she got a good deal on a calendar to use for off requests. But then we realized it was one of those rare Monday first calendars, and it has driven us crazy all year. Only a few more weeks, and we can get back to a real calendar that matches the other three calendars we have to use.
I not only hate Monday-first calenders, I hate the idea of Monday-first calenders. Sunday’s been the first day of the week for what? 6,000 years? And we change it now because some people have a work schedule that lines up differently? Bah. Our culture is disposable enough without changing one of the oldest (minor) aspects of it for no good reason.
This thread is the first time- ever- I’ve ever heard any otherwise sane person state that they consider Sunday the first day of the week. :eek:
As as already been said, Monday is the first day of the week. Saturday and Sunday are the Weekend, and therefore not the start of the week.
Who, me? 
To me, Sunday is the last day of the week, Monday the first. I can’t imagine thinking any other way, even with calendars that have Sunday first. I prefer Monday-first ones, especially since I tend to think of the weekend as a unit and Sunday-first calendars break them up.
Monday. I struggle to get any work done on a Sunday, even when I need to, but on Monday things tend to get going. So, even subconsciously I have Monday as the beginning of the week.