Please show the scripture that called for mankind to observe a sabbath before Moses gave it to the Israelites.
"The Day of the Lord" is not the same as "The Lord's Day". The Day of the Lord is concerning judgment and will be no sabbath, just the opposite.
Gen 26:5 Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.
Exo 20:8 Remember the sabbath day, to keep it holy.
Gal 3:29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Why is the Sabbath such a problem for you? The only reason to have a new Sabbath is if you wish to worship a new god.