Pope Gregory XIII, who gave his name to the Gregorian calendar
In 1582, Pope Gregory XIII decreed that a new calendar be used. This Gregorian calendar has, over the centuries, replaced most other calendars and nowadays it is used practically everywhere. The main reason for the introduction of this new calendar was that calculating the date of Easter proved problematic. Unlike Christmas or All Saints' Day, Easter – the celebration of the resurrection of Jesus Christ – is a moveable feast, which means that its date is not fixed to a particular day of the calendar year but instead changes from year to year.
In early Christianity, different parishes would celebrate Easter on different days. In 325, the Council of Nicaea therefore fixed the procedure for determining the date of Easter Sunday. Easter Sunday would henceforth fall on the first Sunday after the first full Moon of spring (in the northern hemisphere).
1582 was ten days short of a full year
To ensure that the first day of spring, which had been set to 21 March by the Council of Nicaea, would actually coincide with the equinox again, Pope Gregory XIII ordered the advancement of the calendar by ten days in 1582. In some Roman Catholic countries, Thursday 4 October 1582 was therefore directly followed by Friday, 15 October 1582.
Determining the date of Easter is still very complicated, however, as like its predecessors, the Gregorian calendar is based on the solar year of 365 days whereas Easter is defined in relation to the (full) Moon. A large number of medieval mathematicians worked on calculating the date of Easter. Today, we use the slightly modified formula developed in 1800 by mathematician and astronomer Carl Friedrich Gauss to calculate the calendar date.
According to this formula, Easter falls on a Sunday between 22 March and 25 April. This year, the Easter Bunny will be hiding the eggs on 12 April.