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passages of Scripture such as Acts 20:7 ( 7On the first day of the week we came together to break bread. Paul spoke to the people and, because he intended to leave the next day, kept on talking until midnight.), 1 Corinthians 16:2(2On the first day of every week, each one of you should set aside a sum of money in keeping with his income, saving it up, so that when I come no collections will have to be made., Colossians 2:16-17(16Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. 17These are a shadow of the things that were to come; the reality, however, is found in Christ, and Revelation 1:10 indicate that, even during New Testament times, the Sabbath is no longer binding and that Christians are to worship on the Lord’s day, Sunday, instead.

Next, since you go by sola scriptura, I gave you the above, but also, historically, we see that the early Church Fathers compared the observance of the Sabbath to the observance of the rite of circumcision, and from that they demonstrated that if the apostles abolished circumcision (Gal. 5:1-6), so also the observance of the Sabbath must have been abolished.

"We keep the eighth day [Sunday] with joyfulness, the day also on which Jesus rose again from the dead" (Letter of Barnabas 15:6–8 [A.D. 74]).

or from Ignatius of Antioch

"[T]hose who were brought up in the ancient order of things [i.e. Jews] have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord’s day, on which also our life has sprung up again by him and by his death" (Letter to the Magnesians 8 [A.D. 110]).

and Justin Martyr

"But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Savior on the same day rose from the dead" (First Apology 67 [A.D. 155]).

however, let me repeat -- unless you knew of these scriptural reasons and also the historical proof that the earliest Christians celebrated on the Lord's day, Sunday, you could easily be misled as you were and as SeventhDay adventists are misled. Hence, a good question, but one that is easily answered by the above proofs, scriptural and historical --> do you have any proof that Sunday was not and is not the Lord's Day? Or is it something you just made up?