Yes, the crowds did welcome Jesus when He entered Jerusalem less than a week before His death—and yes, they did turn against Him only a few days later. The Bible says they shouted to Pilate (the Roman governor), "Take him away! Take him away! Crucify him!" (John 19:15).
What happened to change their minds? One reason was disappointment; many expected Jesus would declare Himself to be their king, and would lead a revolt against the Romans who were occupying the Jewish nation. In other words, they hoped Jesus would become a political leader.
But Jesus didn't come to set up a new political system. He came instead to change our hearts and save us from our sins by His death and resurrection. He declared during that last week, "My kingdom is not of this world. If it were, my servants would fight. ... My kingdom is from another place" (John 18:36). This deeply disappointed those who hoped He would throw out the hated Roman occupiers.
But the main reason the crowds turned against Jesus was because their hearts had grown spiritually cold. Jesus had been with them three years, performing miracles and teaching them the truth about God. What more evidence did they need that He was the Savior? And yet they rejected Him. Don't let this be true of you, but during this holy season turn your life over to Him.