Is it possible to set the first day of the week to Monday instead of Sunday for the due calendar?
It's not currently possible. There is more work to be done with date formatting, timezones, etc.
It's now possible to choose Monday or Sunday, reportedly: http://help.trello.com/customer/portal/articles/1277216-setting-the-first-day-of-the-week-for-the-calendar
As a Canadian, I'm not big on switching to US spellings just to get Sunday at the start of the week, but at least it's possible now.