Exodus 20:8 "Remember the Shabbat to keep it holy"
HaShem created the world in six days.
Upon completing the creation, He declared the seventh day to be special.
He proclaimed it holy, separating it from the rest of the week by not creating anything on that seventh day - the Shabbat.
The Torah wants us to regard the Shabbat as being holy.
Just as HaShem proclaimed it to be special, we are commanded to recite a special prayer when the Shabbat arrives - the Kiddush, and when it departs - the Havdalah.
These prayers remind us of the holiness and uniqueness of Shabbat.
