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Palm Sunday is a Christian moveable feast that always falls on the Sunday before Easter Sunday. The feast commemorates Jesus' triumphal entry into Jerusalem, an event mentioned by all four Canonical Gospels (Mark 11:1-11, Matthew 21:1-11, Luke 19:28-44, and John 12:12-19).
In many Christian churches, Palm Sunday is marked by the distribution of palm leaves (often tied into crosses) to the assembled worshipers. The difficulty of procuring palms for that day's ceremonies in unfavorable climates for palms led to the substitution of boughs of box, yew, willow or other native trees.
According to the Gospels Jesus rode a donkey into Jerusalem, and the celebrating people there lay down their cloaks in front of him, and also lay down small branches of trees. The people sang part of Psalms 118: 25-26 - ... Blessed is He who comes in the name of the Lord. We bless you from the house of the Lord .
The symbolism of the donkey may refer to the Eastern tradition that it is an animal of peace, versus the horse, which is the animal of war. Therefore, a king came riding upon a horse when he was bent on war and rode upon a donkey when he wanted to point out that he was coming in peace. Therefore Jesus' entry to Jerusalem symbolized his entry as the Prince of Peace, not as a war waging king.