In Psalm 2:7 we find the prophetic words: “I will declare the decree: The Lord hath said unto Me, Thou art My Son: THIS DAY have I begotten Thee.”
Should we ask: “What day?” or “When was Christ officially declared to be the Son of God?” We will find the answer in Acts 13:33:
“God hath fulfilled the same [promises] unto us… in that He hath raised up Jesus again; as it is also written in the second Psalm: Thou art My Son, this day have I begotten Thee.”
So it was at Christ’s resurrection that the “decree” was made and He was “declared” to be the Son of God — “begotten” in the larger sense of the word.
This agrees with what we find in the first chapter of Romans, where St. Paul speaks of God’s good news,
“Concerning His Son Jesus Christ our Lord, who was made of the seed of David according to the flesh; and declared to be the Son of God with power… by the resurrection from the dead” (Rom. 1:3,4).
This is a wonderful truth. It was Christ’s resurrection from the dead in power that proved that He was indeed God the Son. And more wonderful still: it was our death He died at Calvary, so that He might impart to us this everlasting resurrection life.
Thus, because of Christ’s finished work of salvation, those who place their trust in Him are given His resurrection life and “blessed with all spiritual blessings in heavenly places in Christ” (Eph. 1:3). This is their RESURRECTION DAY! source