Easter is celebrated as the first feast in the Christian liturgical calendars; it is attested from the second century. It commemorates the Last Supper, the Passion and Resurrection of Christ, events of which the synoptic gospels locate the course during the celebrations of Passover in Jerusalem, Friday 15 Nissan on the Jewish calendar, while the gospel attributed to John considers the Jesus was crucified on a Friday 14 Nissan. The Easter was celebrated in different ways by the early Christian churches. Some of the early Church continued to celebrate the Last Supper on the day of Passover, especially Syriac churches attached to the Johannine tradition that identified the sacrifice of Christ with the Paschal offering. Others, like the Church of Rome, celebrated Easter on the Sunday following Passover, thus focusing on the Resurrection after Shabbat. In 387, Epiphanius of Salamis demonstrates “the existence of two groups who were celebrating Easter on a fixed date: on one side those who follow the” Jewish myths “, the other a group that set in Cappadocia, celebrate Easter on 8 Kalends of April (25 March). “He said that these people claimed to have found the exact date of the crucifixion of Jesus in Christian sources. However, Epiphanes does not admit that date and indicates that other versions give 15 Kalends of April (March 18) or 10 of these calends (23 March). He added that according to his calculations, it is the 13th of April calends (20 March).
The Hebrew calendar is lunisolar, every month beginning with the New Moon; 14th of Nisan therefore generally corresponds to the Full Moon almost to the day (that is to say, the fourteenth after the new moon visible from Jerusalem closest to the equinox.). The year of the Jewish calendar has 12 or 13 lunar months; for Nissan remains the first month of spring, the intercalation of an additional month was decided by the Sanhedrin when it was necessary to respect the seasons. After the First Council of Nicaea in 325, it was decided to calculate the date of Easter following a fixed rule. Thus, “Easter is the first Sunday after the Paschal Full Moon, the full moon that occurs on or soonest after 21 March (taken to be the date of the equinox)” so the Sunday after the first full moon event during or after the spring equinox. A problem emerged later, is the difference in practice between Western churches and Orthodox churches. The first adopted in 1582 the Gregorian calendar to calculate the date of Easter, while others continue to use the original Julian calendar. The World Council of Churches proposed a reform of the method of determining the date of Easter at a summit in Aleppo (Syria) in 1997. This reform would have eliminated the differences in dates between Eastern and Western churches; she had come into force in 2001, but it failed.
Calculating the date of Easter is quite complex. It is known as computation. There are traditional tables, but also more mathematical algorithms to find it. The first method developed by Carl Friedrich Gauss had some errors: in 1954 (the formula given on April 25 instead of April 18) and in 1981 (April 26, instead of 19 April). Many other mathematicians have since developed other formulas.
Some religious groups choose to practice this ceremony in agreement with the Jewish Passover, that is to say, the day of the Quartodecimanism for the Church of God (Seventh Day) and some Seventh Day Baptists or the day of the Memorial Jehovah’s Witnesses.