A.T. Robertson Commentary Matthew 24:42

"Watch therefore: for ye know not on what day your Lord cometh." — Matthew 24:42 (ASV)

Watch therefore (γρηγωρειτε ουν). A late present imperative from the second perfect εγρηγορα from εγειρω. Keep awake, be on the watch "therefore" because of the uncertainty of the time of the second coming. Jesus gives a half dozen parables to enforce the point of this exhortation (the Porter, the Master of the House, the Faithful Servant and the Evil Servants, the Ten Virgins, the Talents, the Sheep and the Goats). Matthew does not give the Parable of the Porter .