As he rode along,
the people were spreading their cloaks on the road;
and now as he was approaching the slope of the Mount of Olives,
the whole multitude of his disciples
began to praise God aloud with joy
for all the mighty deeds they had seen.
“Blessed is the king who comes
in the name of the Lord.
Peace in heaven
and glory in the highest.”
Some of the Pharisees in the crowd said to him,
“Teacher, rebuke your disciples.”
He said in reply,
“I tell you, if they keep silent,
the stones will cry out!”
--From the Gospel of Saint Luke
Here's a link to an article describing many of the traditions associated with the celebration of Palm Sunday in the Catholic Church.