Easter commemorates the resurrection of Jesus Christ. This annual observance is recognized as a movable feast, meaning its date changes yearly, but it is always celebrated on a Sunday. The decision to fix Easter to this specific day, while allowing its calendar date to vary, is rooted in theological conviction, historical conflict, and complex astronomical calculation. Understanding why Easter is always on a Sunday requires looking at the earliest accounts of the event and the later efforts to standardize the Christian calendar.
The Resurrection and the First Day of the Week
The primary reason Easter is celebrated on a Sunday is the New Testament account of Jesus’ resurrection. The Gospel narratives report that the discovery of the empty tomb occurred early in the morning “on the first day of the week” following the Jewish Sabbath. This day was immediately adopted by early Christians as the weekly focus of worship, often called the “Lord’s Day.” The selection of Sunday was a deliberate move to distinguish the new Christian observance from the Jewish Sabbath, which fell on Saturday. Sunday symbolized the triumph over death and the beginning of a new era. By gathering on the first day of the week, early communities created a weekly celebration centered on the resurrection.
The Council of Nicaea: Standardizing the Date
In the early centuries of Christianity, a disagreement known as the Quartodeciman controversy emerged over the proper date for the annual celebration. Quartodeciman refers to the practice of some churches, primarily in Asia Minor, to observe the Christian Pascha on the 14th day of the Jewish month of Nisan, the day of the Jewish Passover, regardless of the day of the week it fell. Other churches insisted the celebration must always fall on the Sunday following Nisan 14 to honor the day of the resurrection.
This lack of uniformity was addressed at the First Council of Nicaea in 325 AD, convened by Emperor Constantine. The council ruled that Easter must be celebrated on the same Sunday throughout the world, establishing a unified practice. The decision mandated that Easter must always fall on a Sunday, solidifying the theological significance of Sunday as the day of the resurrection.
Determining the Date: The Paschal Full Moon
The Council of Nicaea established the rule that Easter should be the first Sunday after the first full moon that occurs on or after the vernal equinox. This formula was designed to ensure Easter remained connected to the lunar cycle of the original Passover event while still adhering to the Sunday mandate. The date calculation, known as the computus paschalis, does not rely on astronomical observation but on a standardized ecclesiastical calendar.
The Church fixed the date of the vernal equinox as March 21, regardless of the actual astronomical date. The “full moon” used in the calculation is not the true astronomical full moon, but a calculated date called the Paschal Full Moon. This ecclesiastical full moon is determined using a 19-year cycle, known as the Metonic cycle, which approximates the phases of the moon.
The Paschal Full Moon is the first calculated full moon on or after the fixed date of March 21. Easter Sunday is then defined as the Sunday immediately following that Paschal Full Moon. This method ensures that the earliest possible date for Easter is March 22 and the latest is April 25. Western Christian churches utilize the Gregorian calendar for this calculation, while most Eastern Orthodox churches continue to use the older Julian calendar, which often results in them celebrating Easter on a later Sunday.