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.
Zechariah 9:9 prophesied this day. The account of the story can be read in Matthew 21: 1-11
In the Catholic Church, red vestments are used to symbolize the blood Jesus would shed as a result of His entry into Jerusalem.