Palm Sunday is the day we remember the "triumphal entry" of Jesus into Jerusalem, exactly one week before His resurrection (Matthew 21:1-11). Some 450-500 years earlier, the Prophet Zechariah had prophesied, "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).Matthew 21:7-9 records the fulfillment of that prophecy: "They brought the donkey and the colt, laid their clothes on them, and set Him on them. And a very great multitude spread their clothes on the road; others cut down branches from the trees and spread them on the road. Then the multitudes who went before and those who followed cried out, saying: ‘Hosanna to the Son of David! Blessed is He who comes in the name of the LORD!’ Hosanna in the highest!’” This event took place on the Sunday before Jesus' crucifixion.
In remembrance of this event, we celebrate Palm Sunday. It is referred to as Palm Sunday because of the palm branches that were laid on the road as Jesus rode the donkey into Jerusalem. Palm Sunday was the fulfillment of the Prophet Daniel's "seventy sevens" prophecy: " Know therefore and understand, That from the going forth of the command To restore and build Jerusalem Until Messiah the Prince, There shall be seven weeks and sixty-two weeks; The street shall be built again, and the wall, Even in troublesome times" (Daniel 9:25). John 1:11 tells us, "He (Jesus) came to His own, and His own did not receive Him." The same crowds that were crying out "Hosanna" were crying out "crucify Him" five days later (Matthew 27:22-23). ~Got Questions.org