The Bible is clear and simple regarding the Sabbath. The Sabbath was defined at the time when God created the earth. All who chose to obey God and do God's will celebrated Sabbath as the memorial of God's creative work.
The practice was formalized in writing at the time of Moses. The Jews were observing Sabbath at the time of Christ and generally do so today. The observance on Saturday was suppressed by Rome after Rome defeated the Jews in battle. Roman authorities then oppressed and persecuted all Sabbath-keepers, Jews and Christians. Sunday worship replaced Saturday worship over time.
There is no Biblical authority for the change. The last book in the Bible was written about 100 years after the birth of Christ. Roman Sun worshippers celebrated on Sunday before the time of Christ and did not change after the time of Christ. It was the people that claimed to be Christian that changed. The change took place over a period of about 200 years.
The conclusion is clear and simple. Apostate leaders supported the change of the worship day from Saturday to Sunday. They banded together with other leaders to issue official documents, declarations and statements, etc. to make the change official for their followers.
The choice each individual has to make is clear and simple. Do you want to act according to what is in the Bible? Or, do you want to trust in leaders that oppose what is in the Bible?
Read, think, reason, understand and ask God for wisdom.
What SDAs Believe about the Sabbath
Justifying Sunday worship from the Bible (Stephen Davey)