Determining the Date of Easter

It is a recurring topic in the news: Christians from various traditions are striving to agree on a unified date for Easter. But how is it possible that this date is not the same everywhere? Here's a brief introduction to what has proven to be one of the most intricate issues within church tradition.
We all know that the Jewish people celebrate Passover on the 14th of Nisan. In Asia Minor, modern-day western Turkey, Christians also celebrated Christ’s passion, crucifixion, and resurrection on the 14th of Nisan. This meant that the day of the week varied, as the 14th of Nisan is determined by a lunar calendar, which differs yearly compared to the solar calendar. Elsewhere, including in Rome, Easter was celebrated on the Sunday following the Jewish Passover. To summarize:

Originally, the Jewish calendar was based on observations of the new moon. This later changed to a calculated calendar, using simplified computations based on the average positions of the sun and moon, assuming circular orbits and constant speeds. As a result, the 14th of Nisan could sometimes fall before the start of spring. Some Christians would then celebrate Easter on the Sunday after the 14th of Nisan, while others celebrated it on the Sunday following the first full moon after March 21. The situation can be summarized as follows:

With the growing authority of the Bishop of Rome, the Council of Nicaea, convened by Emperor Constantine the Great in 325 AD, decided that Easter would be celebrated on the Sunday after the 14th of Nisan. The Council established that Easter should be observed on the first Sunday following the first full moon after the vernal equinox (when day and night are equal in length). However, since the exact date of the equinox was not precisely calculable in antiquity, the Council did not provide a strict formula for determining it. The general rule was that Easter would not coincide with the Jewish Passover; if it did, Easter would be postponed by a week. The aim was to have a unified celebration of Easter, but disagreements over how to determine the vernal equinox complicated matters. For example, Rome used March 18, while Alexandria used March 21.

By the 2nd century, the celebrations of the Ascension and Pentecost were also linked to Easter. By the 3rd century, the Ascension was celebrated separately on Pentecost, the 50th day after Easter.
In 525, Pope John I commissioned the monk Dionysius Exiguus to establish a calendar beginning with the birth of Christ. Previously, years were counted from the founding of Rome, and before that, they were named after the two ruling consuls. Dionysius also created the so-called Easter table, listing Easter dates for future years. His calculations fixed the start of spring on March 21, making Easter dates vary between March 22 and April 25. At the time, the Julian calendar, introduced by Julius Caesar, was in use. This calendar had 365 days per year, with a leap day every four years, closely but not perfectly aligning with the solar year.
By 1582, the astronomical spring fell on March 11. Pope Gregory XIII implemented calendar reforms, dropping 10 days to realign the calendar, so October 4, 1582, was immediately followed by October 15, 1582. He also decreed that century years would not be leap years unless divisible by 400. This reform improved alignment with the solar year and created what is now known as the Gregorian calendar.
However, the Eastern Orthodox Church rejected the authority of the Pope of Rome and continued using the Julian calendar.

Even among Eastern churches using the Gregorian calendar for fixed-date holidays like Christmas, differences in Easter calculation remain. This is because Easter must fall after the Jewish Passover, calculated using the Julian calendar in the East.
In March 2021, Archbishop Job of Telmessos, representing the Ecumenical Patriarchate of Constantinople, proposed unifying the Easter date for all Christians starting in 2025, marking 1700 years since the Council of Nicaea. His suggestion was to use the astronomical calendar, celebrating Easter on the first Sunday after the spring full moon, aligning with current Western practice. However, it is unclear whether Easter would still be postponed if it coincided with Passover.
Whether Christians will ever celebrate Easter on the same date remains to be seen.