Yahweh God NEVER told anyone to change His seventh day Sabbath to the first day. Furthermore, His days run from sunset to sunset, not from midnight to midnight:
Genesis 1:5 - 5 God called the light day, and the darkness He called night. And there was evening and there was morning, one day.
He called the days He created the First Day, the Second Day, etc. He rested on the 7th Day, and even made it one of His Commandments that we do the same!
Genesis 2:2 - By [Ex 20:8-11; 31:17] the seventh day God completed His work which He had done, and [Heb 4:4, 10] He rested on the seventh day from all His work which He had done.
Exodus 20:11 - "[Gen 2:2, 3; Ex 31:17] For in six days the LORD made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the LORD blessed the sabbath day and made it holy.
Exodus 23:12 - "[Ex 20:8-11; 31:15; 34:21; 35:2, 3; Lev 23:3; Deut 5:13f] Six days you are to do your work, but on the seventh day you shall cease from labor so that your ox and your donkey may rest, and the son of your female slave, as well as your stranger, may refresh themselves.
However, like everything else, man decided to "do his own thing".
Isaiah 53 - 6 All of us like sheep have gone astray, Each of us has turned to his own way; But the LORD has caused the iniquity of us all to fall on Him.
So,WHY did man decide to name the days of the week (which ended up being named after Pagan gods), when the Bible commands us NOT to use pagan deity names? Because, like everything else, Satan made sure that God's commandments were confused and ignored....
Exodus 23:13: and in all things that I have said to you take heed: and make no mention of other elohim, neither let it be heard out of your mouth.
Psalms 16:4: their sorrows shall be multiplied that give gifts to idols: Their drink-offerings of blood will I not offer, nor take their names upon my lips.