Because it is the only weekly, perpetual reminder that the Lord created us. The 7th day Sabbath is the sign of God, His trademark. Every 7 days, every weekend we are reminded who created us and created our world. Thank you.
This is why we call it a Happy Sabbath:
Remember the sabbath day, to keep it holy.
9 Six days shalt thou labour, and do all thy work:
10 But the seventh day is the sabbath of the Lord thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
11 For in six days the Lord made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the Lord blessed the sabbath day, and hallowed it.
Exodus 20: 8-11