Palm Sunday or Passion Sunday is the last Sunday of Lent. Today we remember the Passion of Christ. Jesus entered Jerusalem while the people shouted Hosanna and threw Palm branches in his path. Hosanna is an exclamation of supplication in a moment of emotion. The Palms are a sign of victory and joy.
The people celebrated the Triumphant entry of the King of Kings into Jerusalem. Ironically just a few days later these same people will call upon Pilate to crucify Him.
In the Catholic Church, red vestments are used to symbolize the blood Jesus would shed as a result of His entry into Jerusalem.