Actually the command has changed a few times. If you look at the command as it was given originally in Exodus 20:8-11, the reason God gave for resting on this day was because of CREATION, God created the earth in six days and rested on the seventh. When God gives this command again in the Book of Deuteronomy He changes it. In Deuteronomy 5 no mention is made at all of resting because of creation, but rather because God had saved His people from slavery in Egypt. He gives that as the new reason for observing this command, it was creation but it changed to redemption. Hebrews 4:1-11 speaks of “rest” rest on the seventh day, rest because of entering the promised land, but it says there was also another REST that Old Testament believers were looking forward to, the ultimate REST that is found only in Jesus Christ, He is our REST! So now because the focus is no longer on a day but on a PERSON – CHRIST we can understand why the day changed yet again. In the Old Testament they were always looking forward to the REST they would have in Christ, the REDEMPTION He would bring, that is why they worshiped on the seventh day. But in our day we look back on what has already been accomplished by Christ and we know He accomplished our redemption because He rose from the dead. That is why we worship on the first day of the week. This we see in Acts 20:7; Paul met with God’s people on the First Day of the week (Sunday). If you read more in the Book of Acts you will see he (Paul) evangelized unbelieving Jews on Saturdays, the old Jewish Sabbath. It is now the day unbelievers worship, because Christ is LORD of the Sabbath and He and His redemption is the Focus of our rest.