Genesis 2:3, [And God blessed the seventh day and made it holy, because on that day he rested from all the work of creating that he had done.]
Show me a calendar where Sunday is the seventh day!
Hebrews 4:7, [Therefore God set a certain day, calling it today].....in verse 4 Paul says "somewhere he has spoken about the seventh day in these words".....
If you think that the people of God have somehow missed a day along the trail somewhere, you are gravely mistaken.
Again, God's Calendar is not ours. It doesn't say that Saturday is the seventh day. There is also a scripture about "Woe to you, Pharisee's and Hypocrites"...something about "straining at a nat and swallowing a camel".....
Whether the 7th day is Saturday or Sunday makes NO difference regarding Christ's mercy or salvation. And then there is the issue of worrying about the day used for sabbath, and rejecting the true Church ordained by Christ....