Way back before modern religion came about those who followed the teachings of Jesus were being prosecuted because it was the strong belief of Rome that they were the cause of their downfall the Romans had their own god’s and it was believed that having other worshipers in their midst was turning their god’s against them.
Until one man found the answer Constantine, and honestly if you do some study into him I strongly believe that he was not a sane man, however during one battle it was said that he had a vision of a cross that lead him to victory, henceforth it was decided that to ultimately end years of bloodshed and argument and to appease the god’s or as it would become one God. It seemed like a brilliant political move, combine all the religious moments into one single movement and tie it all under Sunday which I believe was dedicated to a god. It was a brilliant maneuver and it gave Constantine a lot of power.
From that time a lot has changed however Sunday remains and even some parts of the bible are re-translated to support it however just as in the time of Lutheran if you are to actually read the bible in its original form it makes it very clear that the Sabbath is the 7th day and it is when our heavenly father rested. Straight up reading genesis in the Hebrew God must have known that there would be argument surrounding it and therefore the worded the book of genesis very carefully.
With my limited understanding of Hebrew from taking 2 years in college
In the beginning God created the heavens and the earth
On the first day day one God created the light this was the first day day one
On the second day day two this was the second day day two
The Hebrew wording is very specific and exact and it specifically continues to repeat itself leading up to the 7th day as Sabbath when God rested and made it holy it structure a calendar of sorts and identifies the days of the week and repeats them and even sets up a time scheme midnight to midnight
I haven’t studied Greek yet fully however there are many references in the New testament to the 7th day as Sabbath and even when cruicifying Jesus they took time to observe the Sabbath
While I have nothing against Sunday worshippers and I believe that they can get into heaven I also believe that the bible is very clear about the Sabbath and that it has not changed
As John clearly states “In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God.”
John 1:1-2 KJV