Ignoring the religious angle entirely, I've always thought it was weird that people consider Monday the first day of the week, because to me, Sunday and Saturday are at either "end" of the week, hence they are called "weekend" days.

If Monday is day 1, then Saturday is day 6 and Sunday is day 7, so Saturday is no longer a weekend.

(Yes, I know that you can argue it a different way such that it still makes sense with Monday as the first day of the week. But it just feels "wrong" to me, probably because I lived the first 15-20 years of my life without even knowing that anyone else didn't consider Sunday to be the first day of the week. Stuff you grow up with is hard to change.)

(Relatedly, no matter how hard I try, Celsius degrees never feel natural to me. Ditto for kilograms when talking about how much a human weighs. I've managed to get myself comfortable with meters and liters, at least, though I still sometimes have to "translate" in my head for things to make sense to me.)

At any rate, I do wonder when this difference solidified. Was Monday considered the first day of the week in western Europe during the period when Europe was colonizing pre-United-States? If so, when and why did the US (or colonists in the pre-US) switch to Sunday?