in the German speaking part of Europe, a calendar is usually shown with MONDAY being the first day, and both Weekend days SAT and SUN at the end of the calendar.
This should be something that is user specific. So where in the user setting can either the admin or the user himself change the first day of the week from SUN to MON?
Since in one Enterprise account, staff from different countries and continents can work together, this should not be an account setting for all the users, but rather a setting that is user specific.
Out of the same reason, it shouldn't be sheet-based, as staff from several offices can work on the same sheet. Again, it has to be user-specific.