Editor’s Note: This article was first written in 1994 and then appeared in the Word Magazine in 2000.
It should be noted that the article objectively examines the origin, history and methods for dating Pascha. In stating facts about methodology, it is not the author’s intent to propose a revision to the current dating methods. In fact, the author closes his article with a reaffirmation of the current Orthodox Unity in celebrating our most important Holy Day. The last paragraph clearly echoes the sentiment of the First Ecumenical Council that the dating of Pashca should be done, "With one accord and in the same manner".
I’ve heard that the reason the Orthodox usually celebrate the Resurrection later than Protestants and Roman Catholics is because we wait until after the Jewish Passover. This year the Jews observed Passover on March 27. Western Christians celebrated Pascha†  after that, on April 3, so why did we wait until May 1?
Our observance of the Resurrection is related to the “Passover of the Jews”†  in a historical and theological way, but our calculation does not depend on when the modern-day Jews celebrate. The reason why Orthodox and Western Christians celebrate at different times is because we still go by the old Julian calendar in calculating the date of Pascha, even though we go by the new calendar for all the fixed feasts (like Christmas and so on). Protestants and Roman Catholics use the Gregorian Calendar for everything.
Pascha in the Old and New Testaments
The Old Testament specifies that the Passover/Pascha is to be observed on the 14 th day of the first month (alternately known as Abib or Nisan; see Deuteronomy 16.1-7). Being a fixed day on the old Hebrew calendar, it could fall on any day of the week.
According to the Gospel of John, Pascha just happened to fall on a Saturday†  the year that Jesus was crucified. It is important to note that Christ died on the Cross at the very hour the paschal lambs were being slaughtered for the Feast; thus Christ is our Pascha, our Passover Lamb, sacrificed for us. Strictly speaking, then, we must distinguish between the Feast of Pascha (on Holy Friday) and the Feast of the Resurrection (on Sunday); the two are inseparable though distinct.
The date of Pascha in the Early Church
The early Church in the East continued to observe Pascha on the eve of the 14th of Nisan, according the Jewish Calendar, with the Resurrection on the third day, that is on the 15th. That meant that the Resurrection could fall on any day of the week. In Rome and Alexandria, however, the early Christians always kept the Resurrection on a Sunday.
A Problem Situation
In the second century, St. Polycarp, Bishop of Smyrna in Asia Minor, journeyed to Rome to confer with Pope Anicetus regarding the disagreement over the proper date for the celebration of Pascha. Neither was able to convince the other, and they decided that the two practices could coëxist.
The situation was actually messier yet. There existed in practice, because of the way the Hebrew calendar worked, not two but a multitude of dates for the celebration Pascha. Jews and others in the ancient Near East followed a lunar calendar in which each month averaged 29½ days in length. They had twelve months in most years, each month beginning with a new moon. This made the year too short, so an extra, thirteenth month was inserted every two or three years to keep the months in step with the seasons (which depend on the sun rather than the moon).
There were no printed calendars at that time, and no one ever knew exactly how many days there would be in a given month or year. The beginning of a new month was declared when the first sliver of a new moon was sighted in the sky. Of course, observation of the new moon depended on location and weather conditions, thus people in different places often did not start a new month at the same time. Since Pascha was observed on the 14 th of the month† —and that depended on local sighting of the new moon—there was no way for Christians (or Jews, for that matter) to plan a united observance of Pascha.
In the fourth century the Emperor Constantine espoused Christianity and made it not only legal but the favored religion of the Empire. The Church suddenly started growing by leaps and bounds, and he gave public buildings for the Church’s use, but he was perturbed to find out about the different practices regarding the date of Pascha.
The council of Nicæa
Constantine convened the First Ecumenical Council in the city of Nicæa in 325 to unify the date of the observance throughout the newly Christian Empire.†  Unanimously, the bishops gathered at the Council decided to keep the feast on a Sunday. They wanted to retain the symbolism of the Resurrection falling on the day which is both the first day of the week and the eighth day, the Day of the Lord. They agreed that the most important thing was for the Church to demonstrate her unity by celebrating together, whenever she chose to celebrate, without regard to the Jews’ plans. The bishops saw the Christian observance of the Pascha of the Lord on Holy Friday as connected to and in continuity with the Passover of the Old Testament, and they understood that the Resurrection, by definition, follows the Passover. After all, the Church saw herself as the true heir of the Old Testament. She was comprised of both Jews and gentiles, all those who responded to the God of the Old Testament when He came in the flesh.
Following the Council, Constantine sent a letter to all the bishops who were absent to report to them the decisions of the council. The following excerpt of that letter explains some crucial points:
When the question relative to the sacred festival of Pascha arose, it was universally thought that all should keep the feast on one day; for what could be more beautiful and more desirable than to see this festival, through which we receive the hope of immortality, celebrated by all with one accord and in the same manner? It was declared to be particularly unworthy for this, the holiest of festivals, to follow the calculation of the Jews….
The Nicene Formula
The fathers gathered at the First Ecumenical Council decided that the Hebrew calendar had to go. They had to be able to plan ahead and not have to depend on when the local Jewish Rabbi would spot the new moon. They adopted, therefore, a solar calendar based upon the best scientific and astronomical data of the time. In fact they adopted the civil calendar of the Roman Empire which had been promulgated under Julius Cæsar (hence the name Julian Calendar), as refined under Augustus Cæsar.
The Council decreed that the Resurrection would be observed on the first Sunday after the first full moon on or after the vernal equinox (March 21).†  Furthermore, since the best scientific observatories were located in Alexandria at that time, the Council assigned the bishop of Alexandria the responsibility of sending out a letter to all the Church, year by year, announcing in advance when the Resurrection would be celebrated that year. This way, the whole of Christendom was sure to celebrate together a glorious Pascha/Resurrection.
The Current Situation
After a while, it got tedious to send out letters year by year. Instead of making fresh astronomical observations, people just started calculating when the full moon would occur for many years into the future. This actually worked out rather well for a while; small errors in the calculation only showed up when extrapolating for hundreds or thousands of years out. In fact the ancients were aware of the imprecision, but they devised a nineteen-year cycle based on the Julian Calendar which they considered sufficiently accurate for their purposes, over the time period of 50-100 years with which they were concerned.
Unfortunately, we have been using the 19-year cycle in calculating the date of the Resurrection ever since the fourth century without actually checking to see what the sun and moon are doing. In fact, besides the imprecision of the 19-year cycle, the Julian calendar itself is off by one day in every 133 years. In 1582, therefore, under Pope Gregory of Rome, the Julian Calendar was revised to minimize†  this error. His “Gregorian” calendar is now the standard civil calendar throughout the world, and this is the reason why those who follow the Julian Calendar are thirteen days behind.†  Thus the first day of spring, a key element in calculating the date of Pascha, falls on April 3 instead of March 21.
So let’s do our own calculation for the date of the Resurrection for this year (1994) according to the rule of the First Ecumenical Council: the first Sunday after the first full moon on or after the first day of spring.
The Orthodox Church held an important council in 1923. The Churches that were represented at the council, including Constantinople, Alexandria and Antioch, decided to adopt the Gregorian Calendar for all fixed feasts and to continue to use the Julian Calendar for the date of the Resurrection.†  Let us pray that, one day soon, we can rediscover the goal of the First Ecumenical Council, that the whole Orthodox Church might adopt the most precise calendar available, and—much more important—that we might demonstrate our unity by celebrating all our feasts together, “with one accord and in the same manner.”
Carrying a Cross around the church at Holy Friday matins we sing:
Let us not keep festival as the Jews: for Christ our God and Passover is sacrificed for us. But let us cleanse ourselves from all defilement and with sincerity entreat Him: Arise, O Lord, and save us in thy love for mankind!
†  The term Pascha comes from the Hebrew pesah, a yearling lamb that was sacrificed at the Jews’ spring festival. The feast itself came to be called Pascha (or Passover; see Exodus 12.5f.). We should try to use the term Pascha instead of “Easter.” Easter was a spring festival in honor of Eostre, an Anglo-Saxon goddess of fertility and sunrise. Orthodox Christians should use the terms Pascha and Resurrection instead.
†  John 11.55.
†  Recall that a day starts at sunset in both Jewish and Orthodox reckoning, thus Saturday begins at Sunset Friday.
†  By definition, the full moon was on the 14 th day of the month.
†  As well as to deal with the heresy of Arius, a new controversy raging in the Church at the time. He did not like this confusion in the religion which he had just joined.
†  Note that it is redundant to add “after the Passover,” because saying “after the full moon” already takes care of that requirement—since the 14 th of Nisan, according to the old reckoning, had meant at the full moon of the first month in the spring.
†  The errors can be minimized but not eliminated because a year (the length of time it takes the earth to make one revolution about the sun) is not an even multiple of days (the length of time it takes the earth to make one revolution about its own axis). There remains a fraction for which leap years can only partially make up. The Gregorian reform did not invent a new calendar but just introduced the principle of leap centuries: meaning that there would be no February 29 in any year evenly divisible by 100 unless it is also evenly divisible by 400. Thus there will be February 29, 2000, but there will be no February 29 in 2100, 2200 or 2300.
†  That is, when it’s December 25 on the Julian Calendar, it is already January 7 according to the Gregorian revision. The Julian calendar will continue to lose days as time goes on.
†  The Orthodox Church of Finland, however, uses the Gregorian calendar for all its calculations; thus the Orthodox of Finland are celebrating on April 3 this year. The Churches of Jerusalem, Sinai and Eastern Europe, including Russia, Serbia, Romania, on the other hand, continue to use the Julian Calendar for everything.