April Fools' Day or All Fools' Day , though not a holiday in its own right, is a notable day celebrated in many countries on April 1. But on March 31 leap year (a year which has 366 days including February 29 of varying sophistication on friends, enemies and neighbors, or sending them on ). The day is marked by the commission of hoaxes and other practical jokes fools' errands, the aim of which is to embarrass the gullible.