From: Mark 13:24-32
Signs of the end of the world and the coming of the Son of man [continued]
(Jesus said to His disciples,)  "But in those days, after that tribulation, the sun will be darkened, and the moon will not give its light,  and the stars will be falling from heaven, and the powers in the heavens will be shaken.  And they will see the Son of Man coming in clouds with great power and glory.  And then He will send out the angels, and gather His elect from the four winds, from the ends of the earth to the ends of heaven.
The Time of the Destruction of Jerusalem
 "From the fig tree learn its lesson: as soon as its branch becomes tender and puts forth its leaves, you know that summer is near.  So also, when you see these things taking place you know that He is near, at the very gates.  Truly, I say to you, this generation will not pass away before all these things take place.  Heaven and earth will pass away, but My words will not pass away."
 "But of that day or that hour no one knows, not even the angels in Heaven, nor the Son, but only the Father."
24-25. It would seem that at the end of time even irrational creatures will shrink before the Supreme Judge, Jesus Christ, coming in the majesty of His glory, thus fulfilling the prophecies of the Old Testament (cf., e.g., Isaiah 13:10; 34:4; Ezekiel 32:7). Some Fathers, such as St. Jerome ("Comm. in Matthew, in loc.") and St. John Chrysostom ("Hom. on St. Matthew", 77) understand "the powers in the heavens" to mean the angels, who will be in awe at these events. This interpretation is supported by the liturgical use of describing the angels, taken together, as "virtutes caelorum" (cf. "Roman Missal", Preface of Martyrs). But many other commentators think the phrase, like the preceding words in the text, could mean "cosmic forces" or "stars of the firmament".
26-27. Christ here describes His Second Coming, at the end of time, as announced by the prophet Daniel (7:13). He discloses the deeper meaning of the words of the ancient prophet: the "one like a Son of Man", whom Daniel saw and to whom "was given dominion and glory and kingdom, that all peoples, nations and languages should serve Him," is Jesus Christ Himself, who will gather the saints around Him.
28-30. As already pointed out in the note on Mark 13:4, Jesus' disciples, following the ideas current among Jews at the time, could not conceive the destruction of Jerusalem as separate from the end of the world; and, also, there is a connection between the two events, in that the former is a prefigurement of the latter. Our Lord answers His disciples in Mark 13:20 by saying that the destruction of Jerusalem will happen in the lifetime of their generation (as in fact occurred in the year 70, at the hands of the Roman legions). For further explanation of the ruin of Jerusalem as a figure of the end of the world, cf. note on Matthew 24:32-35.
31. With this sentence our Lord adds a special solemnity to what He is saying: all this will definitely come to pass.
God has only to speak and His words come true, only He who is Lord of the Universe has all existence in His power, and Jesus has received from the Father all power over heaven and earth (cf. Matthew 11:27 and 28:18).
32. Referring to this verse, St. Augustine explains ("On the Psalms", 36:1): "Our Lord Jesus Christ was sent to be our Master, yet He declared that even the Son of Man was ignorant of that day, because it was not part of His office as Master to acquaint us with it."
Regarding the knowledge Christ had during His life on earth, see the note on Luke 2:52.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.