Psalm 118 is appropriate for Palm Sunday celebrations. It begins with the call to give thanks to the Lord. Verses 3-18, omitted by the lectionary, are in the voice of the king describing the defeat of Israel's enemy. At verse 19, the king prepares for entry into Jerusalem. We hear words of thanksgiving for what has been done and a call for continued protection.
These ancient words still speak to us and for us.
This is the day that the Lord has made; let us rejoice and be glad in it.
Blessed is the one who comes in the name of the Lord.
Bind the festal procession with branches, up to the horns of the altar. (Caveat: translators disagree on this verse.)
You are my God, and I will give thanks to you; you are my God, I will extol you.
Even if a congregation postpones mention of the arrest, trial, and crucifixion to Holy Week services, we know that they follow this triumphal entry into Jerusalem. Carl Halladay in Preaching through the Christian Year C points this out:
If one applies Psalm 118 to Jesus' entry into Jerusalem, it must be remembered that for the original supplicant, the valley of anguish lay in the past, on the fields of war; for Jesus, the valley lay ahead, within the walls of Jerusalem.