Galatians 4.4-5: But when the fullness of time had come, God sent forth his Son, born of a woman, born under the law, (5) to redeem those who were under the law, so that we might receive adoption as sons. (ESV)
The Old Covenant made by God with the people of Israel through Moses established God’s law. This covenant required sacrifices to be made for the forgiveness of sin. The people of Israel were unable to faithfully satisfy the requirements of the law. But God’s purpose for the law was to teach us and to be our guardian “until Christ came, in order that we might be justified by faith.” (Galatians 3.24)
In the course and implementation of God’s plan, he had set just the right, exact, optimum moment, the fullness of time, to send forth Jesus, who would satisfy the requirements of the law. Jesus was born of a woman and of the Holy Spirit and lived a sinless life. And by his death on the cross Jesus paid the penalty for our sin, so that by faith in Jesus we can receive the salvation provided by his righteousness.
The birth of Jesus two thousand years ago was in the fullness of time set by God’s sovereign timing and plan to establish the New Covenant in Jesus.