I must admit, I have never liked preaching on (or reading) a Palm Sunday text to begin Advent. All three years in the lectionary include this option, but in nine years of pastoral ministry I went this route only once (It was my first year). Why did I not do it again? The conflation of Advent and Holy Week seemed to minimize the uniqueness of the two seasons and blur their narrative distinctions. So, you will not get any grief from me if you pass on the Gospel reading this week and turn to the epistle or the Old Testament. You could also go with the alternate reading from Luke (if you did not preach on Mark’s version last week). 
For those who are not bothered by Palm Sunday in December, there is an angle that could highlight the season of Advent rather than detract from it. It is the unmistakably royal accent in this text. Luke gives us an image of Jesus as King and reigning Lord. This comes, in part, through Old Testament (OT) citations and allusions. Verse 38 quotes explicitly Psalm 118:26, which is a psalm of royal entry. The spreading of the cloaks hearkens back to the proclamation of Jehu’s kingship (2 Kings 9:13). Then there is the colt. Other than Jesus, the colt gets more attention than anything else in this text. It calls to mind multiple OT connections: Solomon entered his coronation on a colt (1 Kings 1:32-40); the promised king, righteous and having salvation, would arrive in Jerusalem on a colt (Zechariah 9:9); the descendant of Judah, from whom the scepter would not depart, was mentioned in connection with his colt (Genesis 49:11).
Fortunately, the reign of this king extends beyond the people of Israel. Luke presents Jesus as one who reigns over not just a nation of individuals, but as Lord over all creation. Consider the colt again. In verse 33, when the disciples encountered the colt’s owners, they raised question about this confiscation of their property (as Jesus had said they would). Luke calls these owners “lords” of the colt (οἱ κύριοι αὐτοῦ). In response, the disciples simply say that the Lord (ὁ κύριος αὐτοῦ) has need of it. The message is clear. Jesus is the colt’s true Lord. His ownership transcends theirs. There is a similar theme in verse 40, where Jesus’ reign over creation is even more explicit. When the Pharisees tried to quiet the crowds and Jesus responded with a striking claim. If the people were to remain silent, the stones themselves would cry out. In this way, Jesus echoes Psalm 96:11-13 and Isaiah 55:12.
This reign of Jesus over all things is a good way to set the tone for Advent. As we prepare to celebrate Christmas, we are reminded that the one born in a manger was born as the King over all creation. We do not see it now (Hebrews 2:7-9), but His reign is cosmic (Ephesians 1:22; 1 Corinthians 15:27). Jesus is Lord, as the basic Christian confession goes, and we prepare for Christmas by preparing for and submitting to His eternal reign.
The sermon could lead to this by recalling Luke’s quotation of Psalm 118:26 and highlighting a connection between Jesus’ birth and His entry into Jerusalem. The multitude of disciples (τὸ πλῆθος τῶν μαθητῶν) who welcomed Jesus into Jerusalem recalls the multitude of the heavenly host (πλῆθος στρατιᾶς οὐρανίου) who announced His birth in the hills outside Bethlehem. Both sang of the peace He brings to the glory of God.
Jesus reigns in peace to the glory of God. This could be the focus for the sermon. His is a different type of peace, of course. And like His reign, it is not clearly or fully seen today, but we get glimpses. We get glimpses in the church when the people of God forgive each other as they have been forgiven. As they seek reconciliation with each other in light of the reconciliation they have received with God. As they live selflessly in love for their sisters and brothers, showing forth the selfless love of God in Christ. In this way, the peaceful reign of Christ manifests itself in the church to the glory of God. For the ways in which your local congregation has failed to glorify God in these ways, the law will both accuse and guide. This is why the preacher must also proclaim the faith-creating promises of God in Christ—His promises to reign, to bring peace, and to glorify Himself through His people.
CSL Scholar: Sermon helps from David Lewis of Concordia Seminary, St. Louis on Luke 19:28-40.
Lectionary Podcast: Sermon notes from Arthur Just of Concordia Theological Seminary, Ft. Wayne, IN, on Luke 19:28-40.