One scholar has called the Book of Zechariah the most messianic, the most truly apocalyptic and eschatological of all the writings of the Old Testament. This scholar is saying that there are more references in the book of Zechariah to the Messiah Jesus and Gods plan in the sending of His Son than all the other Old Testament writings.
Zechariah 9:9 says, "Rejoice greatly, O daughter of Zion! Shout, O daughter of Jerusalem! Behold, your King is coming to you; He is just and having salvation. He is Lowly and riding on a donkey, A colt, the foal of a donkey."
This Old Testament prophecy is referring to the King Jesus peaceful entrance. He came into Jerusalem riding on a donkey. This prophecy was fulfilled when He presented Himself to Israel in the Triumphal Entry written about in Matthew 21:1-11.
This is why we celebrate Palm Sunday.