This passage appears in year C of the RCL for Palm Sunday, most recently April 14, 2019.
Summary: Luke's Gospel records the entry into Jerusalem with some notable absences:
No "Hosanna" and No "Palm Branches." On the other hand, Luke offers us some events the other Gospels miss: stones that cry out and crowds that sound like angels. For me, I will likely focus on how Jesus' word and ministry sanctifies and even transforms things -- transforms disciples, transforms donkeys and even transforms crowds, all into instruments of God's work.
Note on passion Sunday: As culture shifted away from company's offering employees off on Maundy Thursday and Good Friday, church leadership grew concerned that people were not hearing the full passion. This gave rise to Passion Sunday. For many older members this is really hard because they remember Palm Sunday as a day of celebration, almost Easter 1. The congregation where I currently serve actually used to do Passion Sunday on Lent 5, following a pre-Vatican 2 tradition.
απεστειλεν/αποσταλμενοι ('send'; 19:29, 32) It is always worth noting this verb. The disciples are sent! Are we sending our people out each week? It is also worth noting why they are sent out:
ευρον (find, 19:32)
λυσαντες (loose, 19:29)
αγαγετε (lead, 19:29)
While the disciples are instructed regarding a colt, I think we can abstract this rather easily to people: We are sent out to find people, free them and lead them to Jesus, where they will be put to work!
εχει χρειαν ('have need'; 19:31, 34) The Lord has a need! This is really mind blowing. This passage feels like an Old Testament story to me, in that God is sovereign, but the people can rebel; it pushes against easy answers to the question of free will and God's control.
δοξα εν υψιστοις ('glory in the highest'; 19:38) This harkens back to the nativity (2:14), where the angels proclaim "peace on earth" (here peace in Heaven) and "Glory in the Highest!" Jesus is transforming people into angels, into heralds of the good news!
μαθηταις ('disciple'; 19:39) It is interesting how Latin changed the tenor of this word. The word in Greek means student, which implies the key concept is learning of knowledge and wisdom. The word 'disciple' in Latin means student, but I think when we hear it, we associate it with discipline (spiritual disciplines, for example). The disciples were first and foremost students, people seeking to learn from Jesus. They make mistakes, they are rebuked and their flesh is weak. But they follow Jesus, along the way sharing the news and multiplying Jesus ministry.
κραξουσιν ('cry out'; 19:40) This verb does not mean sing, speak loudly or shout. It means cry out in a protesting and even crazy way. Like the crowd will 'cry out' to crucify Jesus; the demons 'cry out' at the sight of Jesus. The stones here are not simply singing a song of beauty and praise, but also of protest.