About Palm Sunday
Palm Sunday commemorates the entrance of Jesus into Jerusalem (Matthew 21:1–9), when palm branches were placed in his path, before his arrest on Holy Thursday and his crucifixion on Good Friday.
Matthew 21:1-9 KJ21
And when they drew nigh unto Jerusalem and had come to Bethphage unto the Mount of Olives, then sent Jesus two disciples,  saying unto them, “Go into the village opposite you, and straightway ye shall find an ass tied, and a colt with her. Loose them and bring them unto Me.  And if any man say aught unto you, ye shall say, ‘The Lord hath need of them,’ and straightway he will send them.”  All this was done that it might be fulfilled which was spoken by the prophet, saying,  “Tell ye the daughter of Zion, ‘Behold, thy King cometh unto thee, meek and sitting upon an ass, and a colt, the foal of an ass.’”  And the disciples went and did as Jesus commanded them.  And they brought the ass and the colt and put on them their clothes, and they set Him thereon.  And a very great multitude spread their garments in the way; and others cut down branches from the trees and strewed them in the way.  And the multitudes that went before, and that followed, cried out, saying, “Hosanna to the Son of David! Blessed is He that cometh in the name of the Lord! Hosanna in the highest!”
This coming of Christ was described by the prophet Zechariah
Zechariah 9:9 NIV
Rejoice greatly, Daughter Zion! Shout, Daughter Jerusalem! See, your king comes to you, righteous and victorious, lowly and riding on a donkey, on a colt, the foal of a donkey.
When Christ would appear in his glory, it is in his meekness, not in his majesty, in mercy to work salvation. As meekness and outward poverty were fully seen in Zion’s King, and marked his triumphal entrance to Jerusalem, how wrong covetousness, ambition, and the pride of life must be in Zion’s citizens! They brought the donkey, but Jesus did not use it without the owner’s consent. The trappings were such as came to hand. We must not think the clothes on our backs too dear to part with for the service of Christ. The chief priests and the elders afterwards joined with the multitude that abused him upon the cross; but none of them joined the multitude that did him honour. Those that take Christ for their King, must lay their all under his feet. Hosanna signifies, Save now, we beseech thee! Blessed is he that cometh in the name of the Lord! But of how little value is the applause of the people! The changing multitude join the cry of the day, whether it be Hosanna, or Crucify him. Multitudes often seem to approve the gospel, but few become consistent disciples. When Jesus was come into Jerusalem all the city was moved; some perhaps were moved with joy, who waited for the Consolation of Israel; others, of the Pharisees, were moved with envy. So various are the motions in the minds of men upon the approach of Christ’s kingdom.
From Matthew Henry’s Concise Commentary