Question: where does it say in the bible that sunday is the first day of the week?
Answer: The Bible does not use the word “sunday” for the first day of the week. That is a pagan concoction to honor their sun god, just as monday is moon-day and so on. That is history. At no time or place in the bible was the first day of the week blessed, hallowed, sanctified, or set aside as special in any way.