In contrast to Christmas, the date of Easter changes every year. We talk about Easter being early or late. Why is that? Because it is in part determined by the cycle of the moon.
In the early church the remembrance of the Resurrection of Jesus was held at the same time as the Jewish Passover. Because the Jews use a lunisolar calendar—a calendar that is principally aligned with the cycles of the moon, but includes some provision to also align it with the sun—Passover was determined partly with reference to the moon. The Passover is celebrated from the 14th day of the month Nissan. In our calendar it falls between 26th March and 25th April.
In the eastern churches, particularly Smyrna and Ephesus and other churches in modern-day Turkiye, this practice of celebrating the resurrection at the same time as the Passover continued. Other churches, importantly those in Rome and Alexandria, celebrated the Resurrection on the Sunday following the Passover. One method emphasised the day of the week, the other the continuity with Passover. The different practices lead to disagreement and controversy. The human element came to the fore in the Church quite early!
But the controversy did not end there. As Church and Synagogue grew apart, many Christians did not want to rely on the Jews to determine the date of their most holy days. And so it was determined to reach consensus on the date of Easter at the Council of Nicea in 325 AD. That council determined that Easter should be celebrated on the first Sunday following the first full moon after the vernal (spring) equinox. In that way the date of Easter was loosened from the date of Passover and yet still in that general time of the year. Not all parts of the Church accepted this and the debate raged for a few more centuries.
But even once that system was widely accepted, disagreement ensued. Everyone agreed that the equinox (when the sun is directly above the equator) should be on the 21 March. But there were now two different calendars in use: the Julian calendar, which slowly has become out of synch with the year as determined by the sun; and the Gregorian calendar, which tries to adjust to the solar year. That meant that the actual equinox was not important, but rather the presumed equinox. That’s why the eastern and western churches now often have Easter on different dates.
There have been efforts to get a common date of Easter by all churches. It would be on the first Sunday after the first astronomical full moon in Jerusalem following the astronomical equinox – that would take the different calendars out of it. But these efforts have not been successful.
And so even the remembrance of Jesus’ death and resurrection is tainted by disagreement and dissension, due to human stubbornness and man’s tradition.