Palm Sunday is known as such because we receive palm fronds to participate in the reenactment of Christ's arrival in Jerusalem. In the Gospels, Jesus entered Jerusalem riding a young donkey, and to the lavish praise of the townspeople who threw clothes, or possibly palms or small branches, in front of him as a sign of homage. This was a customary practice for people of great respect.
Palm branches are widely recognized symbol of peace and victory, which is why they are used on this day. The use of a donkey instead of a horse is highly symbolic, it represents the humble arrival of someone in peace, as opposed to arriving on a steed in war.
The colors of the Mass on Palm Sunday are red and white, symbolizing the redemption in blood that Christ paid for the world.
