It is celebrated in many countries, it's a day to play jokes.
In Italy, France and Belgium people tack paper fishes on each other's back and shout "April Fish!" (in French, Italian and Flemish)
Why it is celebrated? Because in the Middle Ages New Year's Day was celebrated on 25th March.
In the 16th century the calendar changed and New Year's Day was celebrated on January 1st, those who celebrated New Year's day on 1st January made fun on those who celebrate it on April 1st.
More information (in Spanish) here: MUY INTERESANTE.