Be on guard! Be alert! You do not know when that time will come. It's like a man going away: He leaves his house and puts his servants in charge, each with his assigned task, and tells the one at the door to keep watch. "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. If he comes suddenly, do not let him find you sleeping. What I say to you, I say to everyone: 'Watch!'"Today as we consider Christ's first Advent, we have the luxury of of seeing it in hindsight. In our rear view mirrors, we see a wind-up of three or four millennia before the incarnate Son of God burst upon the human scene. Though this is true and many actions of this Advent were set in motion early on, very few saw it coming and fewer still were proactively waiting its appearance. Outside of shepherds "keeping their folks by night", and saints like Anna and Simeon were all but blind to the baby born in Bethlehem.
In the season Christ's Second Advent, it is also possible for us to have such an earthbound focus that we're lulled into a walking spiritual coma. Its for this reason that our Messiah calls for our spiritual alertness.