To the best of my knowledge, ISO 8601 has numbered days of the week with Monday as day 1 since it was made a standard in 1988. This makes sense for business and industry, since Monday is normally the first business or work day of the week.
No thought was given to societal usage outside of business. I now see calendars that start the week with Monday, and they are a little confusing. If those eventually become the standard, I think both Sabbath Day and Resurrection Day worshipers will be somewhat disturbed by it.
There is an update to ISO 8601 scheduled for release this March. To clarify the subject of this thread, can someone tell us if this update somehow makes the Monday equals one interpretation more prominent? I assume Christians will never be affected directly by ISO 8601, but re-ordering the week days on calendars will certainly muddy the waters for us.