Palm Sunday is the first Day of Holy Week. It is when Christians remember how Jesus entered Jerusalem riding on a donkey. It is called Palm Sunday because the people were so happy to see Jesus and knew that he was very important, so they took branches from the Palm and Olive Trees that lined the road. They waved them in the air shouting 'Hosanna' (which means 'God Saves' in Hebrew) and put some of them on the road to make it easier for the donkey to walk on the rocky road.
Today, to remember that day, Christians all over the world go to special Palm Sunday Church Services. They often receive small crosses made from Palm leaves or other local woods. Palm Sunday is sometimes called Passion Sunday as Holy Week reminds us of Jesus's passion or death.