When I was a kid in Canada -- 1950s and 60s -- the only calendar layout that I knew of had Sunday as the start of the week, on the left side of the calendar, and it never occurred to me that a calendar could be laid out differently.
I don't know when I first saw a calendar with Monday starting the week
-- 20 or 30 years ago maybe? -- but it still throws me every time. ("Why is Wednesday where Tuesday should be?")
Is this geographical? (I might have come across a Monday-first calendar in Europe, and my wife (who tends to prefer a Monday-starter) was raised in New Zealand.
Technical, perhaps? (Computer clocks might have been programmed by a nerd harbouring some obscure logical fixation which they want to force the rest of the world to adopt....)
(Yes, I've got too much time on my hands today.)