The Sabbath was the seventh day of the week, Saturday. This day was to be a day of rest for Israel because God ceased from His creation activity on the seventh day ( Genesis 2:3). God blessed it and made it holy ( Exodus 20:11) in that He made it different from the other days of the week for Israel. [source][source][source]
This is the only one of the Ten Commandments not reiterated for the church in the New Testament. Traditionally the church has celebrated the first day of the week as a memorial to Jesus Christ"s resurrection, which event is the ground of our rest ( Romans 4:25). [1][source]