April Fool’s Day and Easter falling on the same day has more historical significance than you may realize.
For many centuries the Roman Catholic Church relied on the Julian calendar to mark when Easter began. Knowing when to celebrate Easter is tied to knowing the day when there are 12 hours of daylight and 12 hours of darkness – also known as the spring equinox.
Knowing the day of the spring equinox is important, as Easter is celebrated on the first Sunday following the full moon after the spring equinox. However, during the 1500s the Roman Catholic Church realized that the spring equinox was being incorrectly calculated.
The Julian calendar calculated that there were 365.25 days in a year. But Catholic astronomers discovered that there are actually 365.242199 days in a year.
The difference sounds insignificant, but the astronomers realized that every 129 years the Julian calendar would be one day out of synchronization with the solar year. This would mean that Easter would be incorrectly celebrated and, therefore, other religious days that year would also be recognized incorrectly.
So, in 1582, Pope Gregory XIII issued a decree ordering that all Christian nations adopt a standardized calendar – the Gregorian calendar. This decree moved the new year from the end of March to the first of January. This caused great confusion among those accustomed to the Julian calendar. The poor saps who were ignorant to the change of New Year’s Day to January 1 and continued to celebrate April 1 as the new year were mocked by the Church as “fools.”
So, in essence, Pope Gregory XIII created April Fool’s Day in 1582.