"Palm Sunday is a Christian feast that falls on the Sunday before Easter. The feast commemorates Jesus' triumphal entry into Jerusalem, an event mentioned in each of the four canonical Gospels.
Christian theologians believe that the symbolism is captured prophetically in the Old Testament: Zechariah 9:9 "The Coming of Zion's King – See, your king comes to you, righteous and victorious, lowly and riding on a donkey, on a colt, the foal of a donkey". It suggests that Jesus was declaring he was the King of Israel to the anger of the Sanhedrin.
According to the Gospels, Jesus Christ rode a donkey into Jerusalem, and the celebrating people there laid down their cloaks and small branches of trees in front of him, and sang part of Psalm 118: 25–26 – ... Blessed is He who comes in the name of the Lord. We bless you from the house of the Lord ....
In Luke 19:41 as Jesus approaches Jerusalem, he looks at the city and weeps over it, foretelling the suffering that awaits the city in the events of the destruction of the Second Temple.
In many lands in the ancient Near East, it was customary to cover in some way the path of someone thought worthy of the highest honour. The Hebrew Bible (2 Kings 9:13) reports that Jehu, son of Jehoshaphat, was treated this way. Both the Synoptic Gospels and the Gospel of John report that people gave Jesus this form of honour. In the synoptics the people are described as laying their garments and cut rushes on the street, whereas John specifies fronds of palm. In Jewish tradition, the palm is one of the Four Species carried for Sukkot, as prescribed for rejoicing at Leviticus 23:40.
Although the Epistles of Paul refer to Jesus as "triumphing", the entry into Jerusalem may not have been regularly pictured as a triumphal procession in this sense before the 13th century. In ancient Egyptian religion, the palm was carried in funeral processions and represented eternal life.
The palm branch later was used as a symbol of Christian martyrs and their spiritual victory or triumph over death. In Revelation 7:9, the white-clad multitude stand before the throne and Lamb holding palm branches. God said "we shall call this day palm Sunday for it is the day of passion"https://en.wikipedia.org/wiki/Palm_Sunday