75 The Son is ignorant, then, of nothing which the Father knows, nor does it follow because the Father alone knows, that the Son does not know. Father and Son abide in unity of nature, and the ignorance of the Son belongs to the divine Plan of silence, seeing that in Him are hidden all the treasures of wisdom and knowledge. This the Lord Himself testified, when He answered the question of the Apostles concerning the times, It is not yours to know times or moments, which the Father has set within His own authority. The knowledge is denied them, and not only that, but the anxiety to learn is forbidden, because it is not theirs to know these times. Yet now that He is risen, they ask again, though their question on the former occasion had been met with the reply, that not even the Son knew. They cannot possibly have understood literally that the Son did not know, for they ask Him again as though He did know. They perceived in the mystery of His ignorance a divine Plan of silence, and now, after His resurrection, they renew the question, thinking that the time has come to speak. And the Son no longer denies that He knows, but tells them that it is not theirs to know, because the Father has set it within His own authority. If then, the Apostles attributed it to the divine Plan, and not to weakness, that the Son did not know the day, shall we say that the Son knew not the day for the simple reason that He was not God? Remember, God the Father set the day within His authority, that it might not come to the knowledge of man, and the Son, when asked before, replied that He did not know, but now, no longer denying His knowledge, replies that it is theirs not to know, for the Father has set the times not in His own knowledge, but in His own authority. The day and the moment are included in the word 'times': can it be, then, that He, Who was to restore Israel to its kingdom, did not Himself know the day and the moment of that restoration? He instructs us to see an evidence of His birth in this exclusive prerogative of the Father, yet He does not deny that He knows: and while He proclaims that the possession of this knowledge is withheld from ourselves, He asserts that it belongs to the mystery of the Father's authority.
We must not therefore think, because He said He did not know the day and the moment, that the Son did not know. As man He wept, and slept, and sorrowed, but God is incapable of tears, or fear, or sleep. According to the weakness of His flesh He shed tears, slept, hungered, thirsted, was weary, and feared, yet without impairing the reality of His Only-begotten nature; equally so must we refer to His human nature, the words that He knew not the day or the hour.
Source: On the Trinity (New Advent)