The answer
The word Sabbath (Heb. Shabbat, Gk. Sabbaton) means "repose," i.e., "cessation" from exertion. The name Sabbath was applied to various great festivals, but principally to the seventh day of the week. Two reasons are given for its observance in Israel: God's resting on the seventh day of creation (Ex 20:8-11; 31:16-17) and Israel's having been "slave in the land of Egypt" and having been brought "out of there by a mighty hand and by an outstretched arm" (Deut 5:15).As the Sabbath commemorates God's rest after the act of creation, the first day of the week speaks of Christ's Resurrection. On the first day, the Lord Jesus Christ was unceasingly active. The seventh day commemorates a finished creation, the first day a finished redemption. Historically, Christians observed Sunday, the first day of the week, as the Christian Sabbath. They note that the Lord Jesus Christ arose on the first day of the week (Mt 28:1) and, thereafter, the New Testament church regularly worshiped on Sunday (Acts 20:7; 1 Cor 16:2; Rev 1:10). This day on which the Lord Jesus Christ arose was called the LORD'S DAY (Rev 1:10)
The Holy Book of Isaiah 66:23 says, "And it shall come to pass that from one New Moon to another, And from one Sabbath to another, All flesh shall come to worship before Me," says the LORD." It does not say that the Sabbath has to be a Saturday.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)