The New Testament begins at Matthew 1:1 and ends at Revelation 22:21. KJV
A Testament is generally written before death. In Jesus case it was written by his life and with the aid of his friends. A Testament takes effect after death, but the death of the Testator does not cancel any part of the Testament.
If you don’t want to keep the Sabbath as Jesus and his early followers did or you don’t want to pray the Lord’s supper now because Jesus hadn’t died yet, that is your choice. However, you are errant if you think these examples of living after Jesus are not part of the New Testament.
Another word for Testament is will. The New Testament of the Holy Bible is exactly that. It shows us the will of God through Jesus’ life, death, and resurrection. In it Jesus is the perfect example of following the Father’s will. And it tells us how we are beneficiaries of God’ will.
When does the New Testament take effect?
You can argue at Jesus’ death.
You can argue at its biblical compilation.
Or you can just accept it begins at Matthew 1:1 and ends at Revelation 22:21.