The days are whatver you make them. The only reason that most plans use a Mon-Sun week is that races are typically held on Sundays - meaning that the final week of the plan ends with race day. Balance the workout load according to your schedule.

The Monday through Sunday approach is probably also due to most people working Monday through Friday and having Saturday and Sunday off for long(er) sessions. The only things I can think of when re-arranging a pre-made schedule would be to schedule your longest days prior to a day off so you can recover; schedule runs and swims for days you work (if you can) because typically they take less than the bike; schedule the bike first and the following day could/should be the run; and schedule your bike sessions for days off.

I agree with everyone above. Many pros are not constrained by the 7 day week format and have cycles that last 5 days or 10 days or whatever. The important thing is to understand how to structure the timing of the training load within the time period.

I have always used Monday to Sunday. I guess it's because Monday always feels like the beginning of a new week, despite what the calendar says.