First, it should be known that the date determined each year for Pascha in both the Western Christian and Eastern Christian religions is inaccurate because of differences in calendar calculations that have occurred over time. That said, there is still a simple way to understand the current calculation.
In simplest terms, according to the church Canons which were established the first 1000 years of Christianity and the Seven Great Ecumenical Councils – specifically, in this case, the First Ecumenical Council held in 325 AD – Pascha (Easter) was to be celebrated the Sunday after the first full moon that occurred after the vernal equinox1 – which effectively followed the end of the Jewish Passover as calculated by the Jewish faith at that time in history.
A few hundred years after Jesus’ Resurrection, the Jewish faith encountered difficulty with the calendar calculation for their Passover. So currently, even the time of the Jewish Passover is not considered accurate at this point in human history.
In 2021, the Jewish Passover was calculated to end at the setting of the sun on Sunday, April 4.2 The first full moon that occurred after the vernal equinox and after the end of the Jewish Passover in 2021 was April 26 according to the Moon Phase Calendar of the Farmer’s Almanac – so Pascha (Easter) for Orthodox Christians would be celebrated the first Sunday after April 26 – which was May 2 in 2021.
Hopefully in the future, our religious leaders will be inspired through the Grace of the Holy Spirit to determine a calculation for Pascha that is closer to that time in history when the Lord’s Resurrection actually occurred.
1Note: Jesus was Jewish and He would have celebrated Passover according to when it was calculated to occur during His time on earth.
2Note: Passover is 8 days long for Conservative and Orthodox Jewish Communities. Some Reformed Jewish Communities keep Passover for 7 days. Reference: https://www.chabad.org/library/article_cdo/aid/3974040/jewish/Why-Is-Passover-Seven-or-Eight-Days-Long.htm