In Psalm 2:7 we find the prophetic words: βI will declare the decree: The Lord hath said unto Me, Thou art My Son: THIS DAY have I begotten Thee.β
Should we ask: βWhat day?β or βWhen was Christ officially declared to be the Son of God?β We will find the answer in Acts 13:33:
βGod hath fulfilled the same [promises] unto usβ¦ 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.β
So it was at Christβs resurrection that the βdecreeβ was made and He was βdeclaredβ to be the Son of God β βbegottenβ in the larger sense of the word.
This agrees with what we find in the first chapter of Romans, where St. Paul speaks of Godβs good news,
βConcerning 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 powerβ¦ by the resurrection from the deadβ (Rom. 1:3,4).
This is a wonderful truth. It was Christβs 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 Christβs finished work of salvation, those who place their trust in Him are given His resurrection life and βblessed with all spiritual blessings in heavenly places in Christβ (Eph. 1:3). This is theirΒ RESURRECTION DAY! source