When does Catholic Easter fall? Because it is celebrated separately from Orthodox, and when they coincide

The last time the Orthodox and Catholic Church celebrated the same day was in 2017. However, this year Catholic Easter falls next Sunday 31 March 2024, while Orthodox Easter on 5 May 2024. Easter is considered a mobile holiday, in the sense that its date is moved within the year. So what is the reason why the two Churches celebrate on a different date the festival of the Lord’s Resurrection? What role do astrological calculations play? Is there any chance that we can celebrate together again in the future? Jews used the lunar calendar based on the circle of the Moon. They celebrated Easter—from the Hebrew word “pesah” meaning “passion” (of the Red Sea under Moses’ guidance to the Promised Land)—on the 14th of the month Nisan, which was the day of the first spring full moon, occurring during the spring equinox or immediately after it. The spring equinox was associated with the celebration of Christian Passover from the very first years after the resurrection of Christ. This was because Christ was resurrected on the first day after the Jewish Passover, which fell that year Saturday (which began then—as did the rest of the days—at 6 on Friday afternoon). Initially, the various Christian local churches celebrated Easter on different dates. The Jewish churches mainly of Asia Minor celebrated it on the day of Christ’s death on the 15th of the Jewish month Nisan (on any day of the week falling), while the national churches preferred the first Sunday – as a resurrection day – after the first spring full moon. Due to these disagreements, the First Ecumenical Council in Nice, under Constantine the Great in 325 AD, decided that Easter will be celebrated on the first Sunday after the first full moon of spring and, if the full moon happens Sunday, then on the next Sunday. In this way, on the one hand, Christian Easter would never coincide with Hebrew, on the other hand, the celebration of Christian Easter was associated with an astronomical phenomenon, the spring equinox and the first full moon of spring (the “Pashalian full moon”). Therefore, to calculate the date of Easter one year, it was sufficient to initially find the date of the first spring full moon and then the first Sunday after this full moon. The First Ecumenical Council commissioned the Patriarch of Alexandria to announce annually to the other churches on Easter day, after the date of the first spring full moon had been calculated, with the help of the astronomers of Alexandria. Error generated by deviation during the solar year The calendar that was in force at the time of the First Ecumenical Synod, was the Julian introduced by Julius Caesar in 45 B.C.E., with the help of the Alexandrian astronomer Sissigen. The latter, based on the calculations of the Hippoarch (who, a century ago with remarkable accuracy, had calculated that the solar year has a duration of 365,242 days), established a calendar, whose years had 365 days, while in every fourth year (the so-called “tense”) added another day. However, the Julian Calendar had a slight deviation, as the duration of the solar year is actually 365,242199 days. Thus, the year of Sosigen is greater than the actual by 11 minutes and 13 seconds. Every four years this small error reaches about 45 minutes, while every 129 years it reaches one day, and the spring equinox is moved continuously earlier. The mistake was accumulated and so while the spring equinox at the time of Christ occurred on March 23, 1582 C.E. had arrived on March 11. That year, Pope Gregory XIII commissioned astronomers Christopher Clavius and Luigi Lilio to promote a calendar reform. October 5, 1582 was renamed October 15, in order to correct the ten-day mistake that had accumulated in the previous 11 centuries, so that the spring equinox returned to March 21, as had happened at the First Ecumenical Council. The New or Gregorian Calendar was adopted by the Catholic states of Europe within the next five years, while Protestants were much delayed. The reaction of the Orthodox Church to the Gregorian Diary was even greater, resulting in the Julian Diary remaining in force in all Orthodox states until the 20th century. The change of the Julian Diary from Gregorian to Greece In Greece the Julian Diary was replaced by Gregorian, beginning on 16 February 1923, which was renamed 1 March. 13 days from 1923 were removed, for in the ten days of the Gregorian and Julian error between 325 AD and 1582 another three days had been added, during about three and a half centuries since the introduction of the Gregorian Diary into the West. Initially the Greek Orthodox Church – unlike the Greek state – retained the Julian Diary, but in 1924 accepted the church calendar to identify with the politician and apply for the immovable holidays. But he did not do anything similar for the Easter Calendar and for mobile holidays, which are still calculated on the basis of Julian or Old Calendar. However, the difference in Easter celebration between Orthodox and Catholic is not only based on the error of the Julian Diary, but also on the error of the so-called “Medonic Circle” of the 5th century BC, which was used by the Christian Alexandrine astronomers and on which basis the Orthodox Church continues to count the dates of the future spring moons. In the 13 days of the wrong Julian spring equinox, the error of the 19-year period, which amounts, from 325 AD to today, to about four to five days, must be added, with the consequence that the Meteoria (or Julian) full moon being estimated four to five days later than the actual one. The Greek Orthodox Church still uses the old Julian Diary and the cycle of Meton to determine the date of Easter. Thus, the Orthodox Easter is often celebrated not on the first Sunday after the full moon, but on the next (as in 2012) or after the second spring full moon (as in 2002 and 2013), instead of the first Sunday after the first spring full moon, as the Council of Nice had designated. When will be the last Easter audience Catholics celebrate Easter according to the rule of the First Ecumenical Synod, but the spring equinox and spring full moon are calculated according to the new Gregorian Calendar, having also taken into account the Meteoric error. Thus, Gregorian – Catholic full moon is much closer to astronomical (often coincides or is only a day away) than Julian – Orthodox. In the 21st century the feast limits of Orthodox Easter are estimated to be from 4 April the earliest to 8 May at the latest. The Catholic Easter limits are from 22 March at the earliest to 25 April at the latest. This means that Catholics will never have Easter in May and Orthodox ever Easter in March. It is jointly celebrated Easter for Orthodox and Catholics, when both Gregorian and Juliani-Metonia Easter full moon fall from Sunday to Saturday of the same week (already after 3 April both full moons), so next Sunday is the common Easter. Thus, Easter celebration is to coincide for both Orthodox and Catholics in 2025 (April 20). At the same time, a joint celebration will be again in the years 2028, 2031, 2034, 2037, 2038, 2041 etc.In total, during the current century Easter will be common 31 years, and every next century this will happen more and more rarely. The last common Easter is estimated to occur in the year 2698, as after 2700—due to the accumulation of the Meteoric error—Julian and Gregorian full moon will never coincide that same week.