Lev. 23:3 "There are six days when you may work, but the seventh day is a Sabbath of rest, a day of sacred assembly. You are not to do any work; wherever you live, it is a Sabbath to the LORD." 

Sabbath is one of Christ's 10 commandments:
Exodus 20:8 "Remember the Sabbath day, to keep it holy..." 

Sabbath day changed by man:
"In the decision to celebrate the weekly festive day of rest on Sunday, (rather than Saturday) it may be that Pagan Roman usages played as large a part as in the fixing of the Christmas festival." (The Catholic Encyclopedia, p. 781, vol. 12.) 

Deut. 4:2 "Do not add to what I command you and do not subtract from it, but keep the commands of the LORD your God that I give you." 

Christ kept the Sabbath:
Luke 4:16 "He went to the synagogue as his custom was, on the Sabbath day." 

As did the Apostle Paul:
Acts 18:4 "He argued in the synagogue every Sabbath and persuaded Jews and Greeks." 

Do as Christ did:
1Pet. 2:21 "To this you were called, because Christ suffered for you, leaving you an example, that you should follow in his steps." 

Sabbath to be kept forever:
Exodus 31:12 "For this is a sign between me and you throughout your generations. 
Exodus 31:16 "...a perpetual covenant..." 
Exodus 31:17 "...a sign forever..." 

Even in the future after Christ's Second Coming:
Isaiah 66:23 "...from Sabbath to Sabbath all flesh shall come to worship before me, says the Lord." 

Heb 4:19 "The promise of entering his rest [the Millenium] remains...so there remains a Sabbath rest (in Greek: 'Sabbatismos') for the people of God." 

Sabbath made for all men:
Mark 2:27 "And he [Jesus] said to them, 'The sabbath was made for man, not man for the sabbath.'"