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Palm Sunday commemorates Christ's triumphant arrival in Jerusalem to the cheers of the crowd.
"The next day the great crowd that had come for the Feast heard that Jesus was on his way to Jerusalem. They took palm branches and went out to meet him, shouting, "Hosanna!" "Blessed is he who comes in the name of the Lord!" "Blessed is the King of Israel!" (John 12:12-13)
Later in that week many of the people in that cheering crowd would be among those shouting that Jesus should be executed.
The second part of the Sunday service commemorates the Passion of Christ.