What's the Originbr of April Fools' Day? The April 1 tradition of pranks has been observedbr for several centuries across a variety of cultures. However, the official origins of br April Fools’ Day is unknown. The most popular theory is that the day wasbr caused by the switch from the Julian calendarbr to the Gregorian calendar in the 16th century. The original calendar saw the New Yearbr beginning on March 25, with celebrationsbr occurring on the first day of April. The switch moved the New Year to January 1,br but many still thought April 1 was the proper daybr to celebrate, hence they were “April fools.” Another theory says the day is relatedbr to the arrival of Spring, when naturebr “fools” people with irregular weather. Historians have also linked April Fools' Day to anbr ancient Roman festival known as Hilaria, whichbr was celebrated by people dressing up in disguises. Whatever its origin may be, April Fools’ Daybr has evolved to have its own lore and rules. Superstition says that playing pranksbr past noon or not responding to pranksbr with good humor will attract bad luck.