The connection between the Jewish Passover and the Christian feast of Easter is real since Christ died on the first Jewish Easter Day. Christ’s death and Resurrection had its figures and types in the Old Law, particularly in the paschal lamb, which was eaten towards evening of the 14th of Nisan. In fact, the Jewish feast was taken over into the Christian Easter celebration; the liturgy (Exsultet) sings of the passing of Israel through the Red Sea, the paschal lamb, the column of fire, etc. Apart, however, from the Jewish feast, the Christians would have celebrated the anniversary of the death and the Resurrection of Christ. But for such a feast it was necessary to know the exact calendar date of Christ’s death. To know this day was very simple for the Jews; it was the day after the 14th of the first month, the 15th of Nisan of their calendar. But in other countries of the vast Roman Empire there were other systems of chronology. The Romans from 45 B.C. had used the reformed Julian calendar; there were also the Egyptian and the Syro-Macedonian calendar. The foundation of the Jewish calendar was the lunar year of 354 days, whilst the other systems depended on the solar year. In consequence the first days of the Jewish months and years did not coincide with any fixed days of the Roman solar year. Every fourth year of the Jewish system had an intercalary month. Since this month was inserted, not according to some scientific method or some definite rule, but arbitrarily, by command of the Sanhedrin, a distant Jewish date can never with certainty be transposed into the corresponding Julian or Gregorian date (Ideler, Chronologie, I, 570 sq.). The connection between the Jewish and the Christian Pasch explains the movable character of this feast. Easter has no fixed date, like Christmas, because the 15th of Nisan of the Semitic calendar was shifting from date to date on the Julian calendar. Since Christ, the true Paschal Lamb, had been slain on the very day when the Jews, in celebration of their Passover, immolated the figurative lamb, the Jewish Christians in the Orient followed the Jewish method, and commemorated the death of Christ on the 15th of Nisan and His Resurrection on the 17th of Nisan, no matter on what day of the week they fell. For this observance they claimed the authority of St. John and St. Philip.
In the rest of the empire another consideration predominated. Every Sunday of the year was a commemoration of the Resurrection of Christ, which had occurred on a Sunday. Because the Sunday after 14 Nisan was the historical day of the Resurrection, at Rome this Sunday became the Christian feast of Easter. Easter was celebrated in Rome and Alexandria on the first Sunday after the first full moon after the spring equinox, and the Roman Church claimed for this observance the authority of Sts. Peter and Paul. The spring equinox in Rome fell on 25 March; in Alexandria on 21 March. At Antioch Easter was kept on the Sunday after the Jewish Passover. (See EASTER CONTROVERSY.) In Gaul a number of bishops, wishing to escape the difficulties of the paschal computation, seem to have assigned Easter to a fixed date of the Roman calendar, celebrating the death of Christ on 25 March, His Resurrection on 27 March (Marinus Dumiensis in P.L., LXXII, 47-51), since already in the third century 25 March was considered the day of the Crucifixion (Computus Pseudocyprianus, ed. Lersch, Chronologie, II, 61). This practice was of short duration. Many calendars in the Middle Ages contain these same dates (25 March, 27 March) for purely historical, not liturgical, reasons (Grotenfend, Zeitrechnung, II, 46, 60, 72, 106, 110, etc.). The Montanists in Asia Minor kept Easter on the Sunday after 6 April (Schmid, Osterfestberechnung in der abendlandischen Kirche). The First Council of Nicaea (325) decreed that the Roman practice should be observed throughout the Church. But even at Rome the Easter term was changed repeatedly. Those who continued to keep Easter with the Jews were called Quartodecimans (14 Nisan) and were excluded from the Church. The computus paschalis, the method of determining the date of Easter and the dependent feasts, was of old considered so important that Durandus (Rit. div. off., 8, c.i.) declares a priest unworthy of the name who does not know the computus paschalis. The movable character of Easter (22 March to 25 April) gives rise to inconveniences, especially in modern times. For decades scientists and other people have worked in vain for a simplification of the computus, assigning Easter to the first Sunday in April or to the Sunday nearest the 7th of April. Some even wish to put every Sunday to a certain date of the month, e.g. beginning with New Year’s always on a Sunday, etc. [See L. Günther, “Zeitschrift Weltall” (1903); Sandhage and P. Dueren in “Pastor bonus” (Trier, 1906); C. Tondini, “L’Italia e la questione del Calendario” (Florence, 1905).]