April Fool's Day or All Fool's Day, holiday of uncertain origin, known for practical joking and celebrated on the first of April. Prior to the adoption of the Gregorian calendar in 1564, the date was observed as New Year's Day by cultures as varied as the Roman and the Hindu. The holiday is considered to be related to the festival of the vernal equinox, which occurs on Mar. 21. The English gave April Fool's Day its first widespread celebration during the 18th cent.