When does your calendar start?

I can’t recall what podcast that I was listening to, most likely one of the relay.fm ones and most likely Analog(ue). The discussion came around to, what day do you consider the start of the week. US calendars typically will have Sunday as the first day of the week. This seems silly as Sunday is the day of rest. Sunday is part of the weekEND. So why would you start the week with Sunday?