Jesus and the twelve were sitting on the Mount of Olives. He knew that His hour had come. He chose His words carefully as He anwered the disciples' questions about the destruction of Jerusalem and His return to earth.
He began talking about the fall of Jerusalem, which happened some forty years later as the Romans invaded Palestine under Titus Adronicus. Then He switched topics and answered the question, "What will be the sign of Your coming and of the end of the age?"
What did Jesus mean when He said, "Therefore, keep watch, because you do not know on what day your Lord will come." When a wife is almost to deliver a child, a caring, thoughtful husband asks, "How do you feel this morning? Could this be the day?" He takes note of his wife's complexion and her disposition, and when he sees the baby bump lower, he knows he had better stay close to home or else he's going to miss the event.
The word Jesus used for "keep watch" is an interesting word. Used of a sentry on guard who must sound the alarm in the event of attack, it was also used in the Old Testament when someone's life was in danger. It is also translated, "Keep your eyes open, be on the alert, be wide-awake about [a given situation]" and so forth.
When Jesus said we who belong to Him are to "be watchful" He meant exactly what the word implies. Keep your eyes wide open, be alert, be knowledgeable and live every day as though Jesus' Second Coming could happen before sunset. The hands on the doomsday clock must be within seconds of the midnight hour. Yes, who knows?
Therefore keep watch because you do not know when the owner of the house will come back - whether in the evening, or at midnight, or when the rooster crows, or at dawn. Mark 13:35