The Feast of the Lord's Entry into Jerusalem (also often called "Palm" or "Palm" Sunday) is celebrated a week before Easter, on the sixth Sunday of Great Lent.
The Jewish people, under the rule of the Roman Empire, expected the Messiah as a political liberator, and it seemed to many that the Miracle Worker, who was able to resurrect the dead and feed five thousand people with a few breads, was the one who would lead the Jews to political freedom. That is why Jesus was greeted as a king with palm branches.
Of those who were then on the streets of Jerusalem, only Christ alone knew what his Mission actually consisted of: that instead of the earthly kingdom, He brings the Kingdom of Heaven to man, instead of deliverance from earthly slavery, He frees man from a much worse slavery - from the slavery of sin. He Alone knew that the path now strewn with palm branches leads to execution on the cross.