April Fool's Day traces its origins to a calendar change in the 16th century when the New Year was moved from April 1st to January 1st, leading some to be labeled 'fools' for not adapting. Over time, the day evolved into a celebration where people play clever, harmless pranks, with different traditions in various cultures such as 'poisson d'avril' in France. The holiday is still observed today as a fun challenge to execute the best tricks, adhering to rules that ensure safety and humor.