Palm Sunday, also called the Triumphal Entry, is one of the Great Feasts of the Orthodox Church, celebrated on the Sunday before Pascha. This year it falls on April 12. Palm Sunday is the day when the Orthodox Church celebrates the entry of Jesus into Jerusalem in the days before the Jewish Passover. A mere few days before His crucifixion, Jesus Christ was received by adoring throngs at his entry into Jerusalem on the back of a young donkey.
The believers meet him, and spread out before him his clothes and olive branches. When He and His disciples approached the city Jerusalem, He ordered them to go to the near-by village, and bring him the donkey and his little who were tied-up in the beginning of the village. If they were asked, they should say that the Lord needs it. When the people knew that the donkey was for Jesus, they did not prevent his disciples. They gave Him the donkey, and He solemnly entered Jerusalem. The news of the resurrection of Lazarus already got ahead and thousands of people went to Bethany to meet him.
You can find the event told in the four Gospels:
I invite you to read more about it on these websites: