In the last message, the chief priest had decided that since people were starting to beleive in Jesus because of Lazarus, it was better that Lazarus must also die. Now, the Jews had decided that both Jesus and Lazarus must die.
John 12:12-19 (ESVST)
12 The next day the large crowd that had come to the feast heard that Jesus was coming to Jerusalem. 13 So they took branches of palm trees and went out to meet him, crying out, “ Hosanna! Blessed is he who comes in the name of the Lord, even the King of Israel!”
On hearing that Jesus was coming to Jerusalem for the Passover feast, the people took palm tree branches and went out to meet him. In Matthew, Mark and Luke, they say that they even took their clothes and laid them on the floor where he was going to pass. They cried out Hossanna! blessed is he who comes in the name of The Lord, the king of Israel. Hosanna! from the Hebrew meaning, means save now, help now, or save we pray thee. It later became know to mean Save and Prosper. So the people were actually asking Jesus to save them as in Psalm 118:25-26 and for Jesus to be blessed. They just added at the end that he was also the king of Israel.
Psalms 118:25-26 (ESVST)
Save us, we pray, O Lord!
O Lord, we pray, give us success!
26 Blessed is he who comes in the name of the Lord!
We bless you from the house of the Lord.
14 And Jesus found a young donkey and sat on it, just as it is written,
15 “ Fear not, daughter of Zion;
behold, your king is coming,
sitting on a donkey ‘s colt!”
16 His disciples did not understand these things at first, but when Jesus was glorified, then they remembered that these things had been written about him and had been done to him.
In Luke Jesus actually sends two disciples to go into the village and collect a donkey. When asked why they did that, they must say that The Lord needs it. This is the same donkey that Jesus sat on. This was a fulfillment of Zachariah 9:9 as quoted on the verse. As in many cases, his disciples only understood these things when Jesus was glorified.
17 The crowd that had been with him when he called Lazarus out of the tomb and raised him from the dead continued to bear witness. 18 The reason why the crowd went to meet him was that they heard he had done this sign. 19 So the Pharisees said to one another, “You see that you are gaining nothing. Look, the world has gone after him.”
The people that were there when Jesus raised Lazarus from the dead, continued to bear witness of the fact. It is because of this reason that the crowd went to meet him. The crowd had heard of what he did. As these things were happening, the Pharisees said to one another that they were not gaining anything in trying to discredit and kill Jesus. The crowds were starting to beleive in him as they had predicted.