Lesson of Naming the 7th Day

"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:
Exodus 20.8-11