Ezekiel 20:11-12

I gave them My statutes and explained My ordinances to them, which, if a man keeps, he will live.  Also I gave them My Sabbaths to be a sign between Me and them, that they might know (without any doubt) that I am the Lord who sanctifies them (separates and sets them apart).

Christians are meant to be set apart.  Our ways and habits are meant to be different from those who are not.  Our observance of Sunday church and resting is a sign between God and us.  By keeping the Sabbath Holy, we continue to honor God and His ordinances.  Sunday is a day to refresh ourselves in the word and God.