Saturday is the 7th day of the week, thus making Sunday the 1st day of the week. The LORD sanctified the Sabbath day and it is a day the LORD commands us to remember that day and keep it holy.
Exodus 20 : 8 – 11
8 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.