Saturday, the seventh day of creation, is the day God rested from His works. It eventually became known as Shabbat or Sabbath, meaning “rest” or “cessation”. In Genesis 2:3, of the 7th Day, it says that “God blessed the seventh day and sanctified it, because in it He rested from all His work which God had created and made.” In Exodus 20:10 God commanded His people under the Law to rest on the seventh day, which the Jews still do up to the present.
The number ‘7’ appears over and over throughout the Bible, over 700 times. In scripture, seven symbolizes completeness or perfection, and is also symbolic of God—there are seven Feasts of the Lord in Leviticus 23, and seven Spirits of God (again, representing perfection, not an amount) in the Book of Revelation.
By observing the Sabbath, the Hebrew people demonstrated submission to God’s revelation and His Law, which is the tutor leading us to Christ (Gal 3:24). The Law shuts up all under sin so that the promise might be given to those who believe in Jesus (Gal 3:22).
Sunday, the first day of the week, is the day of Resurrection (Mt 28:1). Sunday is the day that Yeshua triumphed over death and put an end to the curse of the Law (Deut 27:26 and Gal 3:13). By having faith in Christ, we experience the true rest of which the weekly Sabbath was only a foreshadow (Col 2:16,17; Heb 4:8-10).
The number ‘1’ represents Christ; He was raised on the 1st day of the week, He is the firstborn of all creation, and is the first fruits of those to be resurrected. In Him there is neither Jew nor Greek, but we are one in Him (Gal 3:28). Jesus gave his one “new commandment” the night He was betrayed: Love one another. The Spirit descended the first day of the week, a Sunday (Pentecost is the 7th Sunday after the Feast of First Fruits, or the Resurrection Day). Christians worshiped on Sunday from the earliest times as recorded in history and the Bible (Acts 20:7; 1 Cor 16:2; Rev 1:10).
Friday, the sixth day of the week, is perhaps the crucifixion day or Passover, the day Yeshua was killed, although there is a healthy debate about that. We do know man was created on the 6th day (Gen 1:26-31), and that this number is associated with mankind and judgment—Noah was six hundred years old when the flood came upon the earth (Gen 7:6) and the number of the Beast in Rev 12 is “6-6-6”. Man was to work for 6 days (Exo 20:9) before the Sabbath, so six represents human effort and toiling. When Christ was on the cross, darkness came in the sixth hour (Mk 15:33).
The number ‘6’ represents Man and his fleshly efforts. Islam has chosen Friday as its day of worship. Islam glorifies the number ‘6’ (see this website, for instance). Friday precedes the Sabbath, commanded by the Law, usurping the prerogative of God to require righteousness and then to provide the means to achieve it. Islam is lawless; Paul describes a “man” of lawlessness who will appear before the second coming of Jesus (2 Thes 2:1-3). This describes Islam. It rejects the Law of God revealed by the Sabbath and the Righteousness of God, revealed by the resurrection on a Sunday.