No reason. You can carry on lying, murdering and fornicating. He fulfilled the Law so you're good to go.
As a follower of Jesus
A follower of Jesus would follow His example of living. Including keeping the Sabbath. This weeds out the posers.
Jesus said to keep His commandments of loving God and loving one’s fellow man.
The Ten Commandments hinges on those two principles.
Jesus taught us that He is Lord of the Sabbath and we celebrate HIS resurrection on Domenica.
Jesus gives this law its authentic and authoritative interpretation: "The sabbath was made for man, not man for the sabbath." With compassion, Christ declares the sabbath for doing good rather than harm, for saving life rather than killing. "The Son of Man is lord even of the sabbath."
Jesus rose from the dead "on the first day of the week." Because it is the "first day," the day of Christ's Resurrection recalls the first creation. Because it is the "eighth day" following the sabbath, it symbolizes the new creation ushered in by Christ's Resurrection. For Christians it has become the first of all days, the first of all feasts, the Lord's Day (he kuriake hemera, dies dominica)
>Matthew 28:1: "Now after the Sabbath, as the first day of the week began to dawn, Mary Magdalene and the other Mary came to see the tomb."
Mark 16:2: "Very early in the morning, on the first day of the week, they came to the tomb when the sun had risen."
Acts 20:7: "Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight."
I Corinthians 16:2: "On the first day of the week let each one of you lay something aside, storing up as he may prosper that there be no collections when I come."
Remember Mark 2:27 says, "The Sabbath was made for man, and not man for the Sabbath." and
"One man regards one day above another, another regards every day alike. Let each man be fully convinced in his own mind. 6 He who observes the day, observes it for the Lord, and he who eats, does so for the Lord, for he gives thanks to God; and he who eats not, for the Lord he does not eat, and gives thanks to God," (Rom. 14:5-6).and
"Therefore let no one act as your judge in regard to food or drink or in respect to a festival or a new moon or a Sabbath day 17things which are a mere shadow of what is to come; but the substance belongs to Christ." (Col. 2:16-17).A festival is yearly. A new moon is monthly. A Sabbath is weekly. No one is to act as your judge in regard to this. The Sabbath is defined as the mere shadow of what is to come (Jesus), the reality is Jesus. Jesus is our Sabbath. This is in reference to Isaiah 1:13 13 Bring no more vain oblations; incense is an abomination unto me; the new moons and sabbaths, the calling of assemblies, I cannot away with; it is iniquity, even the solemn meeting.
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Besides "And on the first day of the week, when we were gathered together to break bread, Paul began talking to them, intending to depart the next day, and he prolonged his message until midnight," (Acts 20:7).
as imadrmd1 wrote The people of Christ met originally in the evening pf the Roman midnight-to-midnight day of which the time before sundown was still Sabbath. To the Jew, the evening was the beginning of the sundown-to-sundown first day of the week.
"And upon the first dayof the weekσαββατων (sabbaton), when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight" (Acts 20:7 AV).
For at least 30 years after the resurrection/ascension of Jesus (Pentecost was on the first day of the week, likewise) the disciples met in the evening of the day of which the daylight was still Jewish Sabbath, thereby not violating Jewish sensitivities.
"Upon the first day of the weekσαββατων let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come" (1 Cor. 16:2 AV).
The Corinthians were commanded by Paul to meet on the first of the week and have their tithes ready to be gathered each week.
The whole week is a heptad, a bundle of seven, a week of days, a "sabbath" of days, wherein the last day for Jews was set aside for rest from work. But the evenin of that last day is the first day of a new sabbath of days, hence it is also a (new) sabbath.
If you are a sabbatarian, avvording to the law you musy stay at home, go no place, and do no woek. But also if you wish to thus come inder the Law, you must obey the law, all of it, without fail. And doing so will send you to Hades/Hell in it in any way. You cannot be a Christian and be a sabbatarian.