The Catechism in Questions & Answers

160. What marked the start of the sufferings of Jesus Christ?

The sufferings of Jesus began with His entry into Jerusalem: “Now when they drew near Jerusalem [...] He sent two of His disciples; and He said to them ‘Go into the village opposite you; and as soon as you have entered it you will find a colt tied, on which no one has sat. Loose it and bring it. And if anyone says to you, “Why are you doing this?” say, “The Lord has need of it.”’ [...] Then they brought the colt to Jesus and threw their clothes on it, and He sat on it. And many spread their clothes on the road, and others cut down leafy branches from the trees and spread them on the road. Then those who went before and those who followed cried out, saying: ‘Hosanna! Blessed is He who comes in the name of the Lord!’” (Mark 11: 1-9). – Despite all this rejoicing, Jesus Christ knew that the mood of the people would soon change and that He would have to follow the way of the cross.

The sufferings of Christ are often described with the word ‘Passion’, which derives from the Latin word passio, meaning “suffering”.
“Foal”: a term used to describe a young donkey.

“Rejoice greatly, O daughter of Zion! Shout, O daughter of Jerusalem! Behold, your King is coming to you; He is just and having salvation, lowly and riding on a donkey, a colt, the foal of a donkey.”
Zechariah 9: 9