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.