I always thought it began in conjunction with the crucifixion, death, and resurrection of Jesus. Jesus said, "It is finished."
Until Jesus fulfilled the Law, the Law was to be observed.
The Centurion and his Soldiers is the first Gentile proclamation recorded in the Bible after Jesus died.
28 After this, Jesus knowing that all things were now accomplished, that the scripture might be fulfilled, saith, I thirst.
29 Now there was set a vessel full of vinegar: and they filled a spunge with vinegar, and put it upon hyssop, and put it to his mouth.
30 When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost.
John 19:28-30, KJV
54 Now when the centurion, and they that were with him, watching Jesus, saw the earthquake, and those things that were done, they feared greatly, saying, Truly this was the Son of God.
Matthew 27:54, KJV