The Origin of Orthodox Easter
In 325 AD, the Council of Nicaea decided that Easter would be observed on the first Sunday after the first full moon on or after the March equinox.
The reason for the two different calendars actually came from a miscalculation of astronomy.
The primary goal of creating the Gregorian calendar was to alter the time of Easter.
According to history.com, “In 1582, when Pope Gregory XIII introduced his Gregorian calendar, Europe adhered to the Julian calendar, first implemented by Julius Caesar in 46 B.C. Since the Roman emperor’s system miscalculated the length of the solar year by 11 minutes, the calendar had since fallen out of sync with the seasons. This concerned Gregory because it meant that Easter, traditionally observed on March 21, fell further away from the spring equinox with each passing year.”