Determining Easter Day

What is the Difference Between Catholic and Protestant Easter Dates Versus Greek Orthodox Easter Dates?

Greek Easter takes place on a Sunday, but the date is governed by a system determined by three primary conditions:

The Greek Orthodox Church calculates the date of Easter using the ancient Julian calendar, not the Gregorian calendar that Roman Catholics and Protestants follow.
It must be after the Jewish celebration of Passover.
It must be celebrated on the first Sunday after the first full moon after the Spring Equinox, which occurs in late March.

Determining the Date of Easter
In AD 325, the Council of Nicaea made a significant decision, establishing a formula for dating Easter as the Sunday following the paschal full moon. This full moon falls on or after the spring equinox. In practice, that means that Easter is always the first Sunday after the first full moon that falls on or after March 21. Easter can occur as early as March 22 and as late as April 25, depending on the appearance of the paschal full moon.

The Significance of the Paschal Full Moon
The Council of Nicaea decided that Easter must always occur on a Sunday since Sunday was the day of Jesus Christ’s Resurrection from death. Using the paschal full moon to determine the date of Easter comes from the Jewish calendar. The Aramaic word "paschal" means "pass over," which refers to the ancient Jewish feast day of Passover. God commanded that Passover be celebrated on Nisan the 14^th and Unleavened Bread from the 15^th to the 21^st, using the lunar calendar (Leviticus 23:4-8; Numbers 28:16-25). The next Jewish day began at sundown. Unleavened Bread started at sundown on the night of the paschal full moon when all faithful Jews celebrated by eating their Passover sacrifices in a sacred meal, as Jesus did at His Last Supper with His disciples. Christians call the Thursday immediately before Easter Sunday Holy Thursday, but according to the ancient Jewish calendar, sundown Thursday became the Jewish Friday. Friday was the day of Jesus’s crucifixion, and the very first Easter Sunday was the Sunday after Passover, on the Jewish first day of the week, the first day of the New Creation, and the first day of the Old Creation event.

Approximate Dates for the Paschal Moon
Many Christians erroneously believe that the date of Passover determines when Easter is celebrated; therefore, they are surprised when Western Christians sometimes celebrate Easter before the Jewish celebration of Passover. The Jews no longer celebrate Passover and Unleavened Bread as two separate feasts. Instead, since the destruction of the Jerusalem Temple in AD 70, they only refer to the two feasts as “Passover,” when in actuality, they are celebrating Unleavened Bread on the night of the full moon, which would be Nisan 15^th. John’s Gospel also refers to the two feasts as “Passover,” which may help to determine when St. John wrote his Gospel.

The paschal full moon can fall on different days in different time zones, which can present a problem when calculating the date of Easter. If people in different time zones were to calculate the date of Easter depending on when they observed the paschal full moon, then that would mean that the date of Easter would be different depending on which time zone they lived in. Therefore, the church does not use the exact date of the paschal full moon but an approximation.

For calculation purposes, the full moon is always set on the 15th day of the lunar month. The lunar month begins with the new moon. For the same reason, the Church sets the spring equinox date at March 21, even though the actual vernal equinox can occur on March 20. These two approximations allow the church to set a universal date for Easter, regardless of when you observe the paschal full moon in your time zone.

Different Easter Dates for Eastern Orthodox Christians
Easter is not always celebrated universally by all Christians on the same date. Western Christians, including the Roman Catholic church and Protestant denominations, calculate the date of Easter using the Gregorian calendar, which went into effect in 1582. This more astronomically precise calendar is used throughout the West today in both the religious and secular worlds.

Eastern Orthodox Christians (the Greek and Russian Orthodox) continue to use the older Julian calendar to calculate the date of Easter. The Orthodox Church uses the same formula established by the Council of Nicaea to determine the date of Easter, using the old Julian calendar, dating to the reign of Julius Caesar. Because of the differences in the dates in the Julian calendar, the Eastern Orthodox celebration of Easter always occurs after the Jewish celebration of Passover. Erroneously, Orthodox believers may think their Easter date is tied to Passover, but it isn't. As the Antiochian Orthodox Christian Archdiocese of North America explained in a 1994 article entitled “The Date of Pascha.”

The Theological Controversy
The Council of Nicaea (AD 325) set up a formula for calculating the date of Easter to separate the Christian celebration of Christ’s Resurrection from the Jewish Passover. The Council of Nicaea ruled, while Easter and Passover were related historically, that because Christ is symbolically the sacrificial Passover victim (1 Cor 5:7), the holiday of Passover no longer has any theological significance for Christians. The modern calendar is known as the Christian or the Gregorian calendar. In many countries, this more accurate calendar system began to replace the earlier Julian calendar, which was used from 45 BC until AD 1582.

The Julian Calendar
When Julius Caesar conquered Egypt in 48 BC, he was impressed by the accuracy of the Egyptian calendar and felt the need for calendar reform. The calendar he introduced divided a year into 12 months and contained 365 days, with an extra day every fourth year to account for a solar year's actual length of 365.25 days. Julius Caesar introduced The Julian calendar to the Roman world in 46 BC. It was accurate for 128 years. However, in 1582 AD, the Julian calendar drifted ten full 24-hour days from the actual date. Pope Gregory XIII introduced the Gregorian calendar in 1582 to reform the inaccuracies in the Julian calendar. Catholic countries around the world gradually adopted it. However, the Orthodox Christians continue to use the Julian solar calendar. For calculating religious feast days, the Jews continue to use a lunar calendar consisting of 12 lunar months in a 354 or 355-day cycle. Moslems also use a lunar calendar for their religious calendar called the Hijri or Arabic calendar.

The Gregorian Calendar
The length of 365.25 days per year in the Julian calendar was later proved to be wrong when a solar year was found to be 365.2422 and 365.2424 days in tropical and equinox years. This meant that the Julian calendar erred by 0.0078 days and 0.0076 days in the two cases, amounting to a difference of 11.23 minutes and 10.94 minutes, respectively. The error meant that the Julian calendar had omitted nearly an entire day every 131 years. After many centuries, the Julain calendar had drifted so far that it became inaccurate to calculate the exact seasons and the most important day for the Christians, Easter. Work on the reform of the Julian calendar started in the time of Pope Paul III, with suggestions of the famous astronomer Clavius. Pope Gregory XIII introduced the Gregorian calendar in 1582, which the Universal Church adopted.

The Differences Between the Julian and Gregorian Calendars

The Julian calendar omitted 10 days, and the day following October 4, the day the Gregorian calendar was adopted, was known as October 15, 1582.
In the Julian calendar, it was declared that a leap year could be divisible by 4 but not by 100 or 400.
The Gregorian calendar introduced new laws to determine the date of Easter.
In a leap year, the day before February 25 was selected to add an extra day in the Julian calendar; however, in the Gregorian calendar, the additional day for the leap year was after February 28.