Be guard and alert

This seems to be the principal reason why the day and hour of Jerusalem's destruction, though known by the Father, were not made known to any man, neither to the angels, nor by them; nor to the son of man in the days of his flesh, nor by him when on earth; that his people might be upon their guard, against false Christs, and prophets, and their deception; and watch unto prayer, and in it, lest they fell into temptation, and that day should come upon them unawares:

for ye know not when the time is;
the exact and precise time: for though the people of God had notice of it, and were sensible it was at hand, and did make their escape out of Jerusalem; yet they knew not the exact time, but that it might be sooner or later; and the unbelieving Jews were blinded, and in the dark about it to the very last; (See Gill on Matthew 24:42).

Source: biblestudytools.com