God created the earth in six days, and on the the seventh he rested. If there were nothing else said on the importance of the Sabbath, this should still be enough. What better example to follow that the master and creator of all the earth? If he set a day apart from the others and hallowed it-- shouldn't we?
In Genisis 2:3 it states that "God blessed the seventh day, and sanctified it" and in Exodus 31:13 the Lord explains to the children of Israel that the Sabbath "is a sign between me and you throughout your generations that ye may know that I am the Lord that doth sanctify you." God has made the sabbath holy and set it above the rest as a sign of our cleansing -- if we keep it holy. The sabbath is a perpetual covenant and as we honor that covenant we show for a sign that we are His. Elder Bruce R. McConkie remarked that our Sabbath observance is the great test dividing the righteous and the wicked. So on the Sabbath, what do I do? Which sign do I show to the Lord? You really can tell what a man worships by how he spends his time on Sunday…
The Sabbath day is reminder of God's plan for us, of our opportunity to worship (emulate) him and be cleansed and made holy by him. The symbols of the Sabbath are woven deeply into the scriptures, reminding us time and time again of this doctrine.
The children of Israel were slaves while captive in Egypt and as such did not have the choice to rest on the Sabbath. When their captivity ended they were especially thankful for the Sabbath and their opportunity to honor it.
Through the Atonement of Christ we are delivered from the bondage of death and transgression -- and since Sunday was the third day, the day he came from the tomb, it is the day of the Risen Lord. This is our holy day of remembrance, of the day we were set free. Elder Holland has said that when we do not put away the concerns of the world and honor this day, we show ourselves to be terribly ungrateful. Elder Mark E. Petersen of the Quorum of the Twelve Apostles taught: “Our observance or nonobservance of the Sabbath is an unerring measure of our attitude toward the Lord personally and toward his suffering in Gethsemane, his death on the cross, and his resurrection from the dead."
We are sanctified by the Sabbath when we show for a sign that we are Christ's by making Sunday the best day of the seven. It certainly helps me to think of it this way when I need to weed out the "not quite sunday appropriate activities" from my day. I hope I can be better.