European calendars are typically Mon-Sun, UK/AUS/US/CA etc typically Sun-Sat. Just as the more-common-in-Europe 24 hour times have been seeping into UK for decades, perhaps you're seeing the Mon-Sun calendar doing the same thing?

Might be due to an ISO standard that has declared Monday to be the first day of the week. I strongly disagree: Sunday has always been the first day of the week, and Saturday the last. There was no good reason why this needed to be changed. The new standard didn't fix any problems with how weeks worked, and it breaks historical definitions (sabbath being the last day of the week, for example).

There's a children's song about the days of the week by a singer we like who starts his week on Monday, and my son dutifully corrects him every time he hears the song.

UK is most definitely Mon-Sun

It is now, but that was the phenomenon I was responding to. When I was a kid in the 60s it was most clearly sun-sat

AUS is also Mon - Sun

It is now. Australia is moving towards international norms (thank goodness -- let's hope that happens with climate policy as well!).

When I was a little kid Australia was not metric, still used pounds, shillings and sixpences, and yes, the calendar had Sunday first on the left.

Do your calendars use a Min-Sun scheme? All of mine (US based) are Sun-Sat.

AFAIK, traditional Arabic/Jewish week starts at Sunday Therefore: Ahad (Day 1) for Sunday. Sabbath/Sabt (Day 7) for Saturday.