Church of God International has a letter in its latest The International News written in by a person asking about the Sabbath.
Question: Can you prove to me by Scripture alone that our Saturday is the seventh day? I cannot find one verse that proves Saturday is the seventh day, only by other sources from men, like history books, modern rabbis claiming Saturday has never changed or been broken, but by Scripture alone I cannot link Saturday to the seventh day...
ANSWER: The biblical proof you’re asking for doesn’t exist. One cannot determine by the Bible alone that the day we call “Saturday” is the seventh day of the biblical week. But then, the Bible nowhere teaches that the Bible is the sole source of truth—so by what authority do you establish your “Scripture alone” rule? On what biblical grounds do you throw out examination of historical data as a legitimate means of arriving at important truths? The truth is, the extreme form of sola scriptura you have adopted is nowhere taught in the Scriptures.
Make no mistake, the Bible is God’s unique reve- lation to mankind and therefore must be given first place in our quest for truth, but Scripture nowhere teaches that it is the sole source of all true knowledge.
CGI admits it keeps the day because of traditions of men, not from actual scripture.
The Church of God movement (Armstrongism) has made a big point about now falling for "traditions of men" when it comes to following its god. And yet, for 8 decades it has done just that. Keeping traditions of men, especially Herbert Armstrong's interpretations as "gospel truth".
Because of the witness of the faith traditions of both Jews and Christians, the whole world today knows that the day we call Saturday is the seventh day of the week, and that it’s traceable all the way back to the time of Jesus.