The holiday of Easter is observed throughout the Christian world annually but the date shifts every year due to the way the holiday is calculated – which is astronomical in nature. In most of the modern world, the Gregorian calendar is the standard international calendar and also is used by Roman Catholic and Protestant churches.
The rule established to determine Easter Day is that it is to take place the first Sunday after the Full Moon that occurs after the Vernal Equinox (Spring Equinox). Exceptions are that Easter can never occur before March 22 or later than April 25 in a given year.
Using this rule one can get an idea of when Easter will be celebrated each year.
The calculation of Easter dates back to 325 B.C. where the First Council of Nicaea convened by the Roman Emperor Constantine (which at the time used the Julian calendar.)
The Nicea Council decided the church should observe Easter Sunday on the same date throughout the Christian world, and established rules for calculating it and then constructing tables to make it determinable indefinitely into the future. These tables were revised by the 6th century by Dionysius Exiguus, and in 1582 A. D. by Christopher Clavius at the direction of Pope Gregory XIII who completed a reconstruction of the Julian Calendar. This new calendar is referred to as the Gregorian calendar.
A major difference between the Julian and Gregorian Calendars is the “leap year rule”. Without “leap year rules” our calendar date for the Vernal equinox would continually shift, and our entire calendar would be out of synch with astronomical seasons. Adoption of this Gregorian calendar occurred slowly, but by the 1700’s most of western Europe was using it, and it continued to spread to the bulk of the modern world over time.