Not only did they change the day....they also changed the way the days were reckoned. God said in Genesis 1:5, "And there was evening, and there was morning-the first day.
The Sabbath begins on Friday at "Even"...sunset, and continues through Saturday at sunset. Modern tradition now stipulates that the new day begins at "Mid-night"???
Isn't it amazing....God's way has 12 hours of darkness and 12 hours of light. So natural. Leave it to mankind to confuse such a simple concept!