1 With terror do they hear of the coming of the last day, who will not be secure by living well: and who fain would live ill, long. But it was for useful purposes that God willed that day to remain unknown; that the heart may be ever ready to expect that of which it knows it is to come, but knows not when it is to come. Seeing, however, that our Lord Jesus Christ was sent to us to be our “Master,” He said, that “of the day not even the Son of Man knew,” because it was not part of His office as our Master that through Him it should become known to us.
For indeed the Father knows nothing that the Son knows not; since that is the Very Knowledge of the Father Itself, which is His Wisdom; now His Son, His Word, is “His Wisdom.” But because it was not for our good to know that, which however was known to Him who came indeed to teach us, though not to teach us that which it was not good for us to know, He not only, as a Master, taught us something, but also, as a Master, left something untaught. For, as a Master, He knew how both to teach us what was good for us, and not to teach us what was injurious. Now thus, according to a certain form of speech, the Son is said not to know what He does not teach: that is, in the same way that we are daily in the habit of speaking, He is said not to know what He causes us not to know....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)