62 Moreover Paul, the teacher of the Gentiles, forestalls the impious falsehood, that the Only-begotten God was partially nescient. Listen to his words, Being instructed in love, unto all riches of the fullness of understanding, unto knowledge of the mystery of God, even Christ, in Whom are all the treasures of wisdom and knowledge hidden. God, even Christ, is the mystery, and all the treasures of wisdom and knowledge are hidden in Him. But a portion is one thing, the whole another: a part is not the same as all, nor can all be called a part.
If the Son does not know the day, all the treasures of knowledge are not in Him; but He has all the treasures of knowledge in Him, therefore He is not ignorant of the day. But we must remember that those treasures of knowledge were hidden in Him, though not, because hidden, therefore wanting. As in God, they are in Him: as in the mystery, they are hidden. But Christ, the mystery of God, in Whom are all the treasures of knowledge hidden, is not Himself hidden from our eyes and minds.
Since then He is Himself the mystery, let us see whether He is ignorant when He does not know. If elsewhere His profession of ignorance does not imply that He does not know, here also it will be wrong to call Him ignorant, if He does not know. In Him are hidden all the treasures of knowledge, and so His ignorance is an economy rather than ignorance. Thus we can assign a reason for His ignorance, without the assumption that He did not know.
Source: On the Trinity (New Advent)