We worship on the שׁבּת Sabbath [friday evening until saturday evening]; because our Creator, set the seventh day apart and called it holy (Gen. 2:2-3).
Again at Mt. Sinai our Creator told Moses to, “Remember the Sabbath day and keep it holy” (Ex. 20:8-11). Not once in all of Scripture will you find that the day set apart by our Creator was changed to the first day of the week or any other day.