John Chrysostom

"Watch therefore, for ye know not the day nor the hour." Seest thou how continually He adds this, showing how awful our ignorance concerning our departure hence? Where now are they, who throughout all their life are remiss, but when they are blamed by us, are saying, At the time of my death, I shall leave money to the poor. Let them listen to these words, and be amended. …

Homily on the Gospel of Matthew 78 ↗

Augustine of Hippo

For indeed we know the day and the hour neither of that future time when the Bridegroom will come, nor of our own falling asleep each of us; if then we be prepared for this latter, we shall also be prepared when that voice shall sound, which shall arouse us all.

Catena Aurea by Aquinas ↗

Jerome

Therefore, watch, for you do not know the day or the hour. I always advise the prudent reader not to rely on superstitious interpretations, and not to consider what is said in a fictitious manner, but to consider the previous, middle, and subsequent parts, and to connect everything that is written. From this, therefore, he concludes: Watch, for you do not know the day or the hour. …

Commentary on Matthew ↗

Gregory the Dialogist

Wherefore also a general exhortation to the disciples is fittingly added, when it is said: "Watch therefore, for ye know neither the day nor the hour." Because God accepts repentance after sins, if anyone knew at what time he would depart from this present world, he could have devoted one time to pleasures and another to repentance. But He who promised pardon to the penitent did not promise tomorrow to the sinner. Therefore we ought always to fear the last day, which we can never foresee. …

Forty Gospel Homilies, Homily 12 ↗