What happened on Palm Sunday?

The Gospel of St. Matthew gives us this account:

And when they drew near to Jerusalem and came to Bethphage, to the Mount of Olives, then Jesus sent two disciples, saying to them, “Go into the village opposite you, and immediately you will find an ass tied, and a colt with her; untie them and bring them to me. If any one says anything to you, you shall say, ‘The Lord has need of them,’ and he will send them immediately.” This took place to fulfil what was spoken by the prophet, saying,
“Tell the daughter of Zion,
Behold, your king is coming to you,
humble, and mounted on an ass,
and on a colt, the foal of an ass.”
The disciples went and did as Jesus had directed them; they brought the ass and the colt, and put their garments on them, and he sat thereon. Most of the crowd spread their garments on the road, and others cut branches from the trees and spread them on the road. And the crowds that went before him and that followed him shouted, “Hosanna to the Son of David! Blessed is he who comes in the name of the Lord! Hosanna in the highest!” (Matthew 21:1-9)

Why is Palm Sunday also called Passion Sunday?

Although the Gospel account of Christ’s triumphal entry into Jerusalem is read at the beginning of Mass when the palms are blessed, the Gospel of the Passion is then read at the usual place for the Gospel during the Liturgy of the Word. This marks the solemn beginning of the commemoration of the Lord’s Passion and Death.