Prophecy is sometimes very difficult to understand. For example, what is the Day of the Lord and when will it occur? Obadiah 1:15 says, “For the day of the LORD is near upon all the nations. As you have done, it shall be done to you; your deeds shall return on your own head.”
The Day of the Lord occurs at the Second Coming of Christ when He comes in judgment on the nations for their rebellion against God and mistreatment of God's people. It is a little confusing as your read the prophets to determine which part of their prophecy is going to be fulfilled in the near future and which will be fulfilled during the tribulation or the millennium. Chrysostom first set this concept forth in the 4th Century.
As we study this portion of the Gospel let us remember that part of this prophecy has been fulfilled but other parts remain unfulfilled until the Day of Lord, when Jesus Christ returns to earth physically and visibly.
Jesus seems to indicate to us that the destruction of the temple and of Jerusalem are separate in time from his own coming. In the first section of our passage Jesus tells his disciples what to do when Jerusalem is besieged. Then he explains how his Second Coming will be observed. Beloved, the end is near, therefore, “straighten up and raise your heads, because your redemption is drawing near.” Luke 21:2