At first blush, the Gospel reading for the first Sunday does not appear to have much to do with Advent or Christmas. The story avers to what Evangelicals call the “rapture,” and what biblical scholars refer to as a “mini-apocalypse.”
Perhaps the first thing we should do is locate this passage: it comes from the fifth and final discourse in Matthew’s Gospel – the confrontation in the Temple and the discourse on the last things.
In this passage, Jesus reveals to his disciples that the Son of Man will return like a thief in the night. His unexpected return is accompanied by the sudden disappearance of some going about their daily business. Where two men are working in the field, one will be taken. Of two women working at the mill, one will be taken.
One important theme is that the return of the Son of Man – Jesus – will be both sudden and unanticipated. Jesus even equates this future event to the great flood of Noah’s day, where, according to the story, the deluge took many people completely by surprise. In effect, the suddenness and cataclysmic nature of the Genesis flood story becomes a “type” for the return of the Son of Man:
Jesus said to his disciples:
“As it was in the days of Noah,
so it will be at the coming of the Son of Man.
In those days before the flood,
they were eating and drinking,
marrying and giving in marriage,
up to the day that Noah entered the ark.
They did not know until the flood came and carried them all away.
So will it be also at the coming of the Son of Man.
Two men will be out in the field;
one will be taken, and one will be left.
Two women will be grinding at the mill;
one will be taken, and one will be left.
Therefore, stay awake!
For you do not know on which day your Lord will come.
Be sure of this: if the master of the house
had known the hour of night when the thief was coming,
he would have stayed awake
and not let his house be broken into.
So too, you also must be prepared,
for at an hour you do not expect, the Son of Man will come.”
At first glance it makes little sense to select an apocalyptic reading to initiate the season of Advent. Advent anticipates God-become-man: the birth of Christ. However, If we interpret this reading as a transition from the season of ordinary time (which ends with the Feast of Christ the King – which is itself a celebration of the eventual return of Christ), and to the season of Advent, the reading makes more sense.
Considering that we are in year A, we might want to compare the symbolism in this passage with the symbolism of an Advent story in Matthew’s Gospel – the birth of Jesus during Herod’s reign. We should first recognize that Matthew 24 requires a little imagination to understand. The meaning of this particular passage, where Jesus speaks of himself in the third person, is not as obvious as, say, the Good Shepherd in John 10.
In Matthew 24, Jesus uses deliberately paradoxical language. For example, he reverses roles. Jesus insinuates that the Son of Man will return like a thief-in-the-night. Jesus is not suggesting that the thief will take something that does not belong to him. He is suggesting that the thief’s arrival will be completely unexpected – perhaps unwelcome – to some. Continuing with the reversal of roles (an apparently deliberate rhetorical device), the master of the house is not necessarily someone we should imitate, since he was not prepared for the arrival of the unexpected guest – the thief. 
The irony is that the “thief” and the “master of the house” appear to be references to real people. Let’s go back to chapter 2 of Matthew. Here, the Magi inquire with King Herod (the unprepared master of the house) as to the wherabouts of the mysterious “newborn king” (who would appear to Herod as the thief in the night):
When Jesus was born in Bethlehem of Judea, in the days of King Herod, behold, magi from the east arrived in Jerusalem, saying, “Where is the newborn king of the Jews? We saw his star at its rising and have come to do him homage.” When King Herod heard this, he was greatly troubled, and all Jerusalem with him.
Assembling all the chief priests and the scribes of the people, he inquired of them where the Messiah was to be born. They said to him, “In Bethlehem of Judea, for thus it has been written through the prophet: ‘And you, Bethlehem, land of Judah, are by no means least among the rulers of Judah; since from you shall come a ruler, who is to shepherd my people Israel.'”
In chapter 2 of Matthew, the arrival of the “newborn king of the Jews” is certainly unexpected (and unwelcome!) by Herod and his court. Herod’s failure to discover the identity of this new threat to his authority sends him into a rage:
When Herod realized that he had been deceived by the magi, he became furious. He ordered the massacre of all the boys in Bethlehem and its vicinity two years old and under, in accordance with the time he had ascertained from the magi. Then was fulfilled what had been said through Jeremiah the prophet: “A voice was heard in Ramah, sobbing and loud lamentation; Rachel weeping for her children, and she would not be consoled, since they were no more.”
After considering the story of Herod and the birth of Jesus in Matthew 2, one wonders whether Jesus’ comment –if the master of the house had known the hour of night when the thief was coming,he would have stayed awake and not let his house be broken into. So too, you also must be prepared – is not merely symbolic, but also also a stark warning to the disciples not to be unprepared for the coming of the Messiah, as Herod was when Jesus was born.
 See footnote commentary in NAB bible for Mt 24. footnotes 23 and 25.