The Commandment to keep the Sabbath holy is one of the Ten Commandments found in Exodus 20 where we are instructed to rest one day a week. Notice it says the seventh day is the Sabbath. The seventh day is Saturday. Notice why it says we are to rest. It is a reminder that God is the Creator. If people had remembered that God is the Creator we would not have the lie that crowds God out, the theory of evolution.
Sunday is the first day of the week. People go to Church on Sunday and ignore Saturday because they mistakenly think they are keeping the Commandment. They say they honour Sunday because Jesus rose from the dead on the 1st day of the week but the scripture gives no instruction for that.
Exodus 20:8-11
Remember the sabbath day, to keep it holy.
Six days shalt thou labour, and do all thy work:
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:
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.