Creation was finished in six days, and God rested on the seventh. Churches who meet on Sunday recognize this "eighth day" as a day celebrating the new creation in Christ Jesus (and His resurrection on Sunday).
This is also seen in the Old Testament:
"And in the eighth day the flesh of his foreskin shall be circumcised." (Leviticus 12:3)The sign of the Old Covenant (circumcision) was given on the eighth day of life. This symbolized the boys new life, devoted to God. The ceremonial cleansing (Leviticus 14:10) is similar, the sacrifice is offered on the eighth day, the day he is newly clean. And also the feast of tabernacles (Leviticus 23:36).