The Hebrew word for sanctify
(make holy) has the idea of activities being set apart or devoted to God.1 The seventh day is not only the first day to be blessed, it is also the first object ever to be set apart as holy by God. Indeed, it is the only object of sanctification in the entire book of Genesis.2
3 So God blessed the seventh day and made it holy, because on it God rested from all his work that he had done in creation.