According to the Christian calendar, this is the Sunday before Easter, and marks the first day of Holy Week and the end of Lent.
It is the last day of celebration before recalling the sad events that lead up to His crucifixion. The Gospels tell of Jesus riding into Jerusalem for the feast of Passover upon a donkey, and being hailed by the crowds as the promised Messiah; the one to end the Roman occupation and return the city to the Jews. The crowds that had gathered waved palm leaves as he entered the streets, for it had been prophesised that a king would ride into town upon a donkey.
This event also reminds Christians that Jesus went to Jerusalem to face suffering, a trial, and death; the crowds having turned against him.
Our own churches celebrate Palm Sunday by distributing palm crosses to all members of our congregations.