America was conquered and colonized by Europe, so has a European heritage. In Europe Sunday was a rest day at the start of each week, so they would start their week clean (and yes, because of Christianity because they would be blessed on Sunday and confess in church to get rid of their sins). Later the Saturday was added as a work free day, now in some countries another day is added although it's not specific (people can chose which days they want to work).

In Europe they switched to a Monday as the first day of the week, as people tend to start their week with work on Monday and companies also see Monday as the first day instead of a Sunday. Just because we switched doesn't mean everyone switched. And the US usually holds on to old customs, like imperial measuring system.

Most things in society come from Christianity. Like most holidays, how we count years, many values and standards, etc. Christianity is in the roots of our society. It's a large part of the building blocks in society, the roots from history. I'm happy it's not that big of a deal anymore these days, although in my country it's much less than in the US for example.

But to blame Christianity for you not changing the default first day of the week settings I believe is a bit strange.

It's like the use of . And , notations in numbers, or how you write dates (first the day then the month, or the other way around). There are some standards per region / country but usually you can just change these settings.

Are you also angry the days are named after Roman gods?

By the way, originally in Christianity the holy day was a Saturday, the Saturday Sabbath (originating from Judaism). In paganism the holy day was the day of the sun, namely Sunday. But to avoid a religious war with pagans, emperor Constantine gradually changed the holy day for Christians to a Sunday, among other things, to blend both religions together instead of clashing. It's called the Constantine's Edict (321 CE): Emperor Constantine issued an imperial law declaring Sunday ("the venerable day of the Sun") a day of rest for all citizens.

So maybe you should blame paganism or emperor Constantine.

Or just change the settings.

I'm an atheist, I dispise religions and cults (same thing in my eyes) but your post feels like you're searching to blame them for anything, in this case something stupid, while religion still is the cause for so much awful shit in the world. So why not focus on the real religion caused issues instead?