The Day of the Innocents has a dark history from the Gospel of Matthew and the Massacre of the Innocents by Herod the Great, the Roman-appointed King of the Jews. Herod ordered all infants under 2 years old in Bethlehem to be killed because he was afraid that the baby Jesus was the newborn King of the Jews as had been foreseen. The infant Jesus turned out to be in Egypt with Mary and Joseph so Herod had been tricked. This day became a day to trick your friends. and thus followed the tradition of tricking friends on that day.
In Catalonia, on of the most common pranks is the one that consists on placing a "llufa" on someone's back. A "llufa" is a man-shaped pieced of paper which is stung on the back of an "innocent".
Go out and practice it. Have fun!!