The Gregorian calendar reform in 1582 is often linked to theories about the origin of April Fools' Day.
Pope Gregory XIII introduced the reform to correct drift in the Julian calendar and restore the calendar's alignment with the seasonal year, especially for calculating Easter. In the first adopting countries, Thursday, October 4, 1582, was followed by Friday, October 15, removing 10 calendar dates.
One traditional theory holds that confusion over calendar dates helped create April Fools' customs. People who continued celebrating New Year's around the older springtime date could supposedly be mocked by those who accepted the newer calendar. Historians do not regard this as proven, however; the festival's origin remains uncertain.
A related mix-up is France's 1564 Edict of Roussillon, which officially shifted the start of the year to January 1. That earlier change is also frequently connected with the tradition, while a clear 1561 Flemish reference shows that no single origin theory explains every early example.