The same Sabbath Day established by The Church and Blessed Constantine. (and don't start the Adventist Doctrine, because I neither believe it, nor give it credibility!)
The Sabbath Day "established by The Church and Blessed Constantine" (who was so blessed that he wouldn't accept baptism until he lay on his deathbed) does not accord with the Sabbath Day written in stone by God Himself, observed by the Son of God, or kept by His apostles. Ergo, you are not the "the closest to what existed then" on that point alone.
Neither did Yeshua or His disciples bow to any image, or entreat the patriarchs and prophets (their saints) to pray for them--indeed, Yeshua did not even say to pray to Himself, but always directed prayer to the Father in His Name.
Did you ever hear of the one established by God? Genesis 2:3