Palm Sunday is a celebration of Jesus entering into Jerusalem.
"When they drew near Jerusalem and came to Bethphage on the Mount of Olives, Jesus sent two disciples, saying to them, "Go into the village opposite you, and immediately you will find an @#!*% tethered, and a colt with her. Untie them and bring them here to me. And if anyone should say anything to you, reply, 'The master has need of them.' Then he will send them at once." This happened so that what had been spoken through the prophet might be fulfilled: "Say to daughter Zion, 'Behold, your king comes to you, meek and riding on an @#!*% , and on a colt, the foal of a beast of burden.'" The disciples went and did as Jesus had ordered them. They brought the @#!*% and the colt and laid their cloaks over them, and he sat upon them. The very large crowd spread their cloaks on the road, while others cut branches from the trees and strewed them on the road. The crowds preceding him and those following kept crying out and saying: "Hosanna to the Son of David; blessed is he who comes in the name of the Lord; hosanna in the highest." And when he entered Jerusalem the whole city was shaken and asked, "Who is this?" And the crowds replied, "This is Jesus the prophet, from Nazareth in Galilee." Matthew 21: 1-11
This event is told in all four Gospels. The event is celebrated by the blessing and distribution of palm branches, a solemn or simple procession into the Church before the start of the Mass.
In Jerusalem the people were showing honor to the King of kings. This is what we are doing still. Jesus loves us so much and wants to be King in our lives because He loves us. Just like a parent wants the best for their children, so does Jesus want the best for us. So, we honor Him because he has done so much for us, for our good. There is a good reason for remembering the first Palm Sunday and celebrating Palm Sunday today; to show Jesus He is our Lord and King.