Majority of calendars in Japan are starts from Sunday but IMO now people think week starts from Monday. It's relevant to whether people work (or went to school) on Saturday or not (so two consecutive holidays). Some research shows that old ages (who worked on Saturday) tend to think it starts from Sunday.

Calendars tend to follow traditions, and the difference isn't much matter. Also computer apps are tend to follow the US culture since US dominates the industry.