"The days of creation are sufficiently designated by being numbered, but the day that completes (and celebrates) creation is honored by having a name. The name of the seventh day is "Sabbath."
Thus a double purpose is served.
---By the naming of the seventh day it is distinguished from all other days,
---and by the numbering of the others without naming them the fact that the Sabbath is a definitely recurring day is made prominent."
E.J.Waggoner
Remember the Sabbath day, to keep it holy.
Six days shalt thou labor, 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.
Exodus 20.8-11