The job applications for everyone, not just the Lost Office, ask “Are you willing to work weekends and holidays?”
No surprises there.
And if you put down that you will work Saturday and not Sunday (or vise versa) for religious reasons and you hire them you did so knowingly.
Back when I was a little cog in the Mega-Corp machine I had a few of these cases come up. I would always say that what you agreed to you agreed to. Had one boss who really wanted to break it so I sent all the data up to legal and they came back with, what you agreed to you agreed to.
This should not even had to get to SCOTUS. This was the industry standard.