John 21:3

Simon, therefore, comes to fish. For since neither was He with them continually, nor was the Spirit yet given, nor they at that time yet entrusted with anything, having nothing to do, they went after their trade.

Having then nothing to do, they went to their fishing, and this same they did by night, because they were greatly afraid. This Luke also mentions; but this is not the same occasion, but a different one. And the other disciples followed, because they were henceforth bound to one another, and at the same time desired to see the fishing, and to bestow their leisure well.

(Tract. cxxii) If the disciples had done this after the death of Jesus, and before His resurrection, we should have imagined that they did it in despair. But now after that He has risen from the grave, after seeing the marks of His wounds, after receiving, by means of His breathing, the Holy Ghost, all at once they become what they were before, fishers, not of men, but of fishes. We must remember then that they were not forbidden by their Apostleship from earning their livelihood by a lawful craft, provided they had no other means of living. For if the blessed Paul used not that power which he had with the rest of the preachers of the Gospel, as they did, but went a warfare upon his own resources, lest the Gentiles, who were aliens from the name of Christ, might be offended at a doctrine apparently venal; if, educated in another way, he learnt a craft he never knew before, that, while the teacher worked with his own hands, the hearer might not be burdened; much more might Peter, who had been a fisherman, work at what he knew, if he had nothing else to live upon at the time. But how had he not, some one will ask, when our Lord promises, Seek ye first the kingdom of God and His righteousness, and all these things shall be added unto you? (Matt. 6:33) Our Lord, we answer, fulfilled this promise, by bringing them the fishes to catch: for who else brought them? He did not bring upon them that poverty which obliged them to go fishing, except in order to exhibit a miracle.

(Hom.) The fishing was made to be very unlucky, in order to raise their astonishment at the miracle after: And that night they caught nothing.

In the night time before the presence of the sun, Christ, the Prophets took nothing; for though they endeavoured to correct the people, yet these often fell into idolatry.