The best-known account of Christ’s birth (Luke 2) starts with the words, “And it came to pass.”
Paul used similar words:
Galatians 4:4 (NKJV)
But when the fullness of the time had come,
God sent forth His Son.
God the Father had been preparing for this moment from before time. The whole creation was preparation. Adam, Noah, Abraham and Moses were preparation. The shepherd king, David, was preparation. The Exodus and the exile were preparation.
Not too early, not too late, but at exactly the right time—