I have known for years a few Christian denominations in Eastern Europe celebrate Good Friday and Easter one week later than most Christians every year. But because Jesus was crucified and died during Passover, Crucifixion Day and Resurrection Day can't be celebrated one week after Passover. They must all be celebrated the same weekend everywhere Christians live. Scripture is very clear about this, even if you can't figure out which day of the week Jesus died on.
Edit: Sorry about the duplicate thread.
Firstly, as
@prodromos @dzheremi and my other Orthodox friends can explain, the Pope lacked the authority to unilaterally change the Calendar and thus the Paschalion (the method for calculating the date for Easter defined at the Council of Nicea) and so most Eastern Christians, with the exception of the Assyrian Church of the East, the Finnish Orthodox Church, the three Syriac Orthodox jurisdictions in India, and the Armenians outside of the Holy Land, celebrate Pascha (Easter) according to the Julian Calendar rather than the Gregorian Calendar, with the largest Eastern and Oriental Orthodox denominations* using them exclusively. There are also Eastern Orthodox Old Calendarists, mainly Greek and Romanian, like the Genuine Orthodox Church, which will not enter into communion with any Orthodox church that is in communion with a church using the Gregorian Calendar for any purpose, for example, to set the date of feasts not connected to Pascha (thus, they are not in communion with any canonical Eastern Orthodox Church, which they perjoratively call “world Orthodoxy,” and the one thing they hate more than the New Calendar is anything connected to ecumenism, ecumenical dialogue and ecumenical reconciliation; I understand and sympathize with their concerns, but the Moscow Patriarchate under Metropolitan Hilarion Alfeyev has done a brilliant job working with other denominations and also the late evangelist Billy Graham, who assisted them greatly in decades past, with their explicit blessing, by using his international reputation to gain admittance to the USSR to preach the Gospel to large crowds during an era when Orthodox priests were banned by the Communist government from preaching or catechizing the youth).
Now, the theory of the Paschalion, the method for calculating the date of Easter, is that it should always occur on the first Sunday after the vernal equinox. We can no longer rely on Judaism to tell us when Passover is, because after the destruction of the Temple and the devastation of Jerusalem and expulsion of its Jewish population following the Bar Kochba revolt, as any Karaite Jew will tell you, the Rabinnical Jews moved to a fixed calendar system and as a result the dates of Passover no longer were consistent with what they had been during the era of Second Temple Judaism. Unfortunately we do not know if the Karaite method, which is based on observations of the Barley crop and lunar cycles is correct either, and also changes to the genetics of barley which happen naturally over time, like with other plant and animal species, and the ability of fluctuations in the weather and other external environmental factors to influence the growth of the plant, make its sprouting unreliable as a time measurement.
This rules out Quartodecimianism as the basis for calculating Pascha; it also rules out relying on any surviving branch of Judaism. Thus we have the Paschalion, or the Computus, which represents a best-effort compromise to get us as close to the anniversary of the Passion and the Resurrection of our Lord as possible.
Now unfortunately for the Julian Calendar, it turns out the length of the year is shorter than 365.25 days. However, as the rotation of the Earth slows, the Julian calendar will eventually become more accurate, surpassing the Gregorian calendar in a few thousand years. The Orthodox also argue that Pope Gregory (I forget which one, not Pope Gregory I however, who is venerated as a saint by the Eastern Orthodox as Pope Gregory Dialogos, and who made remarks which, along with canons 6 and 7 of the Council of Nicaea, are contrary to the doctrine of Papal supremacy) over compensated, thus resulting in Gregorian Easter as celebrated in the West in some occasions aligning either with the Rabinnical Jewish passover, that the Church and the Karaite Jews regard as incorrectly calculated, or preceding the vernal equinox, both of which are considered error conditions if we follow the computational method of the Paschalion as adopted at the Council of Nicaea.
However those churches using the Gregorian calendar, which initially excluded but later included the Protestants, do so on the basis of Christian unity and convenience. In officially Christian countries where Good Friday is a holiday, calendar standardization is also of great benefit In Finland, for example, the Finnish Government regards both the Lutheran and Orthodox churches as state religions and wants them to celebrate Pascha on the same day, and since Finnish Lutherans outnumber Finnish Orthodox on a 9:1 ratio (although worldwide, the Eastern Orthodox are the second largest denomination and the Lutherans the fourth; the Roman Catholics are obviously the largest and the Anglicans are in no. 3) they will use the Gregorian calendar. Conversely in Greece, Pascha is dated based on the Julian calendar as are other holidays connected to it such as Clean Monday, the first day of Lent in the Orthodox tradition.
*The largest Eastern Orthodox denomination, or jurisdiction in Orthodox parlance, is the Russian-Ukrainian-Belarussian Orthodox Church under the Moscow Patriarchate, with 220 million members, not counting those in its autonomous churches in Bessarabia, the Baltic States, China, Japan, Korea, and the Russian Orthodox Church Outside Russia, of which I am particularly fond (and which has raised much money for the relief of refugees from Ukraine, given that in addition to having many American, English and other converts, ROCOR also has many Ukrainians and all Orthodox churches are open to people of all ethnicities), and the largest Oriental Orthodox jurisdiction is the Ethiopian Tewahedo Orthodox Church, with 40 million members, which is unique in several respects, being almost entirely descended from Jewish converts (the majority of Ethiopian Jews converted in the early fourth century, and those that did not, who are known as the Beta Israel, mostly fled to Israel with the help of the IDF and Mossad after the Derg communist regime rose to power in the 1970s, strangled Emperor Haile Selassie for refusing to renounce Christianity and become an atheist, and began a genocide against Jews, because after Stalin fell out with Trotsky, Communism developed a noticeable anti-Semitic streak despite its veneration of Karl Marx), having the oldest continually used system of music notation in the world, being the only church that preserved 1 Enoch and Jubilees in its Scriptural canon, and also more likely than not has in its possession the Ark of the Covenant.
