"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)
Here the prophet Zechariah describes the 1st coming of Jesus over 500 years before it occurred. He correctly refers to Him as, "having salvation." Therefore, when Jesus did enter Jerusalem riding on a donkey, as prophesied, the people cried, "Hosanna!" or, literally, "Save now!" (Matthew 21:9). They were hoping Jesus had come to save them from the Roman government and establish His kingdom on earth now.
Luke's gospel tells us as Jesus drew near to the city "He wept," because the people did not know the scriptures well enough to understand the reason for His first coming (v.41-44). While those who did (scribes, chief priests Pharisees), sought to kill Him out of fear and jealousy.
Later, during the week of passover after Pilate had publicly stated he could find no fault in Jesus, no crime worthy of death he asked the crowed, "What shall I do with Jesus who is called Christ?" The chief priests and the elders persuaded the multitudes that they should destroy Jesus (Matthew 27:22), so they cried out, "Let Him be crucified!" Some of these voices were more than likely the same ones crying "Hosanna" earlier. How quickly the people were swayed to follow false leadership.
Note: His first coming was to die for the sins of the world, to be the perfect sacrifice for you and for me. His second coming will be much different.
The Truth: "But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up." (2 Peter 3:10)
