Mark 13: The Day of the Lord
Before we consider what the Word has to say to us in this ch, we need to consider it in it's larger context in the Gospel of Mark. Ryrie sees the theme of this book to be Mark's presenting Jesus to the Reader as Christ the Servant. He see's the book divided into two main areas, divided at Mark 10:45, the pivotal and central vs to Mark's Gospel:
Mark 10:45  "For even the Son of Man did not come to be served, but to serve, and to give His life a ransom for many."
The first half of Mark (Mark 1-10) presents the Service of the Servant; then, at the beginning of ch 11, we see Jesus Trimphal entry into Jerusalem, a city which had been waiting for this very event for over 500 years, ever since Zechariah cried out in 520 BC: 
Zec 9:9  Rejoice greatly, O daughter of Zion! Shout in triumph, O daughter of Jerusalem! Behold, your king is coming to you; He is just and endowed with salvation, Humble, and mounted on a donkey, Even on a colt, the foal of a donkey.
With this entry begins the second half of Mark: The Sacrifice of the Servant. Starting with that Tiumphal entry on Sunday, Mark takes us day by day through the last Week of Jesus ending with His resurrection. In this 2nd half, Jesus has entered Jerusalem to die, knowing that He is headed for the Cross, knowing He has such precious little time left before His final act leading to a salvation bought at such a horrible terrible price that the Son of God had to die, to split apart that triune relationship, leaving a grieving Father in Heaven, and a broken hearted Son screaming out His abandonment hanging on a bloody cross. 
[Oh how we fail to appreciate the inconcievable restraint God the Father exercized that terrible day, that He should stand by while miserable wretches such as we mocked, tortured and then murderd with a hammer and railroad spikes His very Only Begotten Son; and He did NOTHING! He who blasted Sodom and Gomorrah into sub-atomic particles, Who holds the very essense of reality together with His own hands, should restrain Himself from our utter annialation, and let us slaughter His precious Son. What LOVE, that it should have the power to restrain His righteous fist from smashing all we are.] 
Starting in the 20th vs of In ch 11 and going to the end of ch 13, Mark shows us Jesus teaching on the Tuesday before His death. He taught on faith (v20-26), about His authority (v27-33), concerning the religious leaders of Israel (the vine-growers) rejection of God's prophets including the Messiah, God's Son, and their ultimate destruction (12:1-12) [some say occured in the destruction of Jerusalem in 70AD], concerning taxes (v13-17), concerning the concept of resurrection (v18-27), concerning the greatest commandments (v28-34), concerning His deity (v35-37), concerning pride (v38-40), and concerning giving (v41-44). Then in ch 13 He gives His final lesson concerning the future, what I call the Day of the Lord. It is also called by many The Olivet Discourse, because Jesus spoke it on the Mount of Olives.