Hi frogoon,
Well, it's probably well to realize 'what' the ISO is. It is an organization established my man to order standardization. That's not quite the same as God speaking. I would think that a born again believer would easily understand that the world, in general, is not going to follow the teachings or commandments of God. I think the Scriptures themselves make that point pretty clearly.
So, you're welcome to argue whether God's word describes Sunday as the first day of the week or the works of men describe Monday as the first day of the week. It isn't going to make any difference to God. If you, on the other hand, are having a difficult time reconciling that what the world says, does and chooses isn't usually in line with what God says, does and chooses, then I'd suggest you look into the heart of your faith. Is your faith in God or in man?
Oh, and BTW, Sunday has never been nor is not now, the Sabbath.
God bless,
In Christ, ted