Feel free to make a case though scripture.
Scripture mentions
two Sunday gatherings, both at 1 Cor 16:1-2, and in the breaking of bread
on Sunday (Acts 20:7). This, combined with the universal practice of Sunday observance of the Lord's Resurrection Day among the earliest Christians, proves what was the apostolic tradition as taught by the apostles personally to their churches (2 Thess 1:15; 1 Cor 11:2).
"Stand firm and hold to
the traditions which you were taught,
whether by word of mouth or by letter from us."
"I praise you because you remember me in everything
and hold firmly to the traditions, just as I delivered them to you"
In the year 100AD, Ignatius, Bishop of Antioch states:
Ingatius, Bishop of Antioch (AD 100)
"Those who were brought up in the ancient order of things 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 - whom some deny, by which mystery we have obtained faith, and therefore endure, that we may be found the disciples of Jesus Christ, our only Master - how shall we be able to live apart from Him, whose disciples the prophets themselves in the Spirit did wait for Him as their Teacher? And therefore He whom they rightly waited for, being come, raised them from the dead (St. Ignatius: Letter to the Magnesians; Ch 9)
This simple statement by a bishop of one of the apostles' churches echoes that which was stated in an even earlier historic document, The Epistle of Barnabas:
Epistle of Barnabas
"Finally He [God] says to them: I cannot bear your new moons and Sabbaths. You see what he means: It is not the present Sabbaths that are acceptable to me, but the one that I have made; on that Sabbath day, which is the beginning of another world. This is why we spend the eighth day in celebration, the day on which Jesus both arose from the dead and, after appearing again, ascended into heaven."
And that statement is followed by many other statements on the subject by the earliest Christians of the apostolic age. Justin Martyr, in about 150 AD says: "On Sunday, we meet to celebrate the Lord's supper and read the Gospels and Sacred Scripture, the first day on which God changed darkness, and made the world, and on which Christ rose from the dead."
And this Sunday observance of the Lord's Resurrection Day continued for centuries without interruption. As Eusebius, bishop of Caesarea writes around the year AD 300: "The day of [Christ's] light...was the day of his resurrection from the dead, which they say, as being the one and only truly holy day and the Lord's day, is better than any number of days as we ordinarily understand them, and better than the days set apart by the Mosaic law for feasts, new moons, and Sabbaths, which the apostle [Paul] teaches are the shadow of days and not days in reality" (Proof of the Gospel 4:16:186).
And so things continued through all the ecumenical councils of the early Church, and even on through the protestant reformation.
The traditions were known by the real, historical practice of the first christians everywhere---which was Sunday observance of the Lord's Resurrection Day. This day, which was always distinct from the Mosaic Sabbath, was the premier day of Christianity,
for Christ's Resurrection was the premier day of all history.