Some date the festival back to Noah's mistake of sending out a dove from the ark before the flood water had gone down - it was a fool's errand to try to find land when the earth was still covered in water and fish. In fact in France children stick fish on each other's back, though the French 'poison' (fish) may originally have been 'passion' in memory of Christ's passion and how people mocked him on the cross.
Possibly the most likely explanation dates back to 1582 when Pope Gregory XIII ordered the calendar to be changed, so that the new year would begin on March 25 instead of January 1. Anyone who forgot that the date of the new year had changed would be the victim of a practical joke on April 1 - the day when the old new year festivities ended. Since then in England people have been playing incredible practical jokes on each other, including the media. But don't forget, April Fool's tricks are only valid till 12.00 in Britain.
April 1 is celebrated in many countries, including Russia where they call it Yomorina (as in 'humour'), Turkey Bir Nisan, and in India the festival of Holi, where children play tricks such as putting salt in tea and firing water pistols at each other. In parts of South and Central America they celebrate Innocent's Day (December 28) which commemorates when King Herod killed the children. Nowadays children play jokes on adults.
