‘The next day a great crowd who had come to the Feast, when they heard that Jesus was coming to Jerusalem, took branches of palm trees and went out to meet him crying, “Hosanna (save now)! Blessed is he who comes in the name of the Lord, even the king of Israel”. And Jesus having found a young ass, sat on it, even as it is written, ‘Do not be afraid, daughter of Zion, behold your king is coming, sitting on an ass's colt'.'
The next day Jesus rode into Jerusalem on an ass's colt, to be greeted by the festive crowds waving palm branches who, with nothing better to do, regularly welcomed Passover visitors in this way. Many weary travellers would have been greeted in this way on arrival and would have found cheer and strength from the words as they reminded them of their future hopes. This explains why the Romans took little notice.
But while this was a regular greeting to pilgrims to the Feast (see Psalms 118:25-26 from which the words were taken), it was no doubt intensified because Jesus was a popular teacher, and because what had happened to Lazarus had increased Jesus' reputation. They were cries of expectancy for the future David, but not necessarily directly related to the one they were shouting to. When a large crowd are shouting out in fervour there are many renderings of the same theme, so we may expect that one or other of the Gospel writers will refer to the ones specifically remembered by their sources. John here draws out the reference to the King of Israel. Jesus was indeed entering as King and Messiah, even though the crowds were not necessarily all aware of it. We have already learned of their uncertainties (John 7:12; John 7:26-27; John 7:31; John 7:40-43; John 10:24). Had the Romans actually seen in these greetings the public ascription of Messiahship to Jesus they would soon have stepped in. Their numbers were heavily increased at Passover time and they were always on the watch for any hint of insurrection.
The waving of date palm branches was a common practise at national celebrations in Israel (see Leviticus 23:40). Palm branches had become a national symbol (compare the Jewish histories 1Ma 13:51; 2Ma 10:7), and they appeared on coins that the Jewish nationalists produced during the war with the Romans in 66-70 AD.
John only mentions the ass so as to draw attention to the fulfilment of prophecy. The prophecy itself is illuminating. ‘Rejoice greatly, Oh daughter of Zion, shout, Oh daughter of Jerusalem, behold your king comes to you. He is just and bringing deliverance, lowly, and riding on an ass, even on a colt the foal of an ass. And I will cut off the chariot from Ephraim, and the horse from Jerusalem, and the battle bow shall be cut off, and he will speak peace to the nations, and his dominion will be from sea to sea and from the River to the ends of the earth' (Zechariah 9:9-10).
Here was not a picture of the great insurrectionist, but one of the humble bringer of peace and deliverance to the world. And that is how Jesus wanted people to see Him.