I will make it simple.
Sunday started out as the first day.
Saturday used to be Savvato, or Sabbath. So it used to be the last day.
Jesus is said to have been born on the first day of the week, or Sunday.
Eventually people started to worship on Sunday instead of Saturday.
As such, to keep with tradition as the last day being the day or rest and prayer, the last day of the week was changed to Sunday.
By doing that, Monday was made into the first day.
Officially Sunday is the first day but unofficially Monday is the first day and because of this you will see some calenders go by Monday being the first day.
Why we go by what the bible says is because every gov't in eastern Europe, which discovered the Americas, is founded by the bible. That is to say that most laws and rules and what is considered good and wrong came from the Bible. Killing was never wrong before the Bible said so.
hope i helped,
s