Palm Sunday is the start of Holy Week - the last week before Jesus' death and resurrection. There was a lot of celebrating on Palm Sunday. The people in Jerusalem were waving palm branches, putting those branches and their coats on the ground to make a path for Jesus. They were cheering and saying:

Hosanna to the Son of David! Blessed is he who comes in the name of the Lord! Hosanna in the highest! -Matthew 21:9