“At that time people will see the Son of Man coming in clouds with great power and glory. And he will send his angels and gather his elect from the four winds, from the ends of the earth to the ends of the heavens.
“Now learn this lesson from the fig tree: As soon as its twigs get tender and its leaves come out, you know that summer is near. Even so, when you see these things happening, you know that it is near, right at the door. Truly I tell you, this generation will certainly not pass away until all these things have happened. Heaven and earth will pass away, but my words will never pass away.
“But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father. 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 their 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.
Mark 13:26-36 (NIV)