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Why the Date of Easter Is So Complicated.

Stained glass window image of the Resurrection of Christ.
The Resurrection.4

Easter 2019 is celebrated on Sunday, April 21. In 2018 it fell on April 1. Why can’t it just stay put like Christmas and other normal holidays?

Why the Date Changes. In Western churches, Easter is celebrated on the first Sunday following the first Full Moon after the Vernal Equinox.1

Dependence on the phases of the Moon by itself makes the dates change. Astronomically, the time from one full moon to the next is about 29.5 days.2 Thus, with 12 lunar cycles in a year, the date of the full moon in any month shifts about 11 days from year to year. In 2019, for example, Boston has full moons on March 20 and April 19, while in 2018 they were on March 1 and March 31.3 Then there is the complicating factor that the full moon must be after the Vernal Equinox, which for these purposes is March 21.

Putting all these factors together, Easter can fall on any Sunday from March 22 to April 25.5

Why the Calculation Is Complicated. Easter commemorates the resurrection of Jesus Christ, just as Christmas celebrates his birth. Nothing suggests that the actual date of one event is better known than the other. Yet Christmas dependably comes on December 25 every year, while Easter depends on the phases of the Moon and the start of Spring. Why did early Christians choose such a complex formulation for one holiday while sticking with a simple calendar date for the other?

The answer depends on which calendar you look at. The Bible says Jesus was crucified at the beginning of the Jewish holiday of Passover. Passover begins on the 15th day of the Jewish month of Nisan. Since the Jewish calendar is based on lunar cycles, the date of Passover changes from year to year just like that of Easter. Thus, apart from always being celebrated on a Sunday, the annual date of Easter is constant on a lunar-based calendar.

Why the connection between the dates of the crucifixion and Passover was considered so important is a question for historians and theologians, but there actually are many other holidays that are not observed on the dates of the events they commemorate. Presidents’ Day, for example, originally was tied to George Washington’s birthday, February 22. Today, Federal law puts it on the third Monday in February. When Memorial Day was first celebrated after the Civil War, pains were taken to ensure that its observance did not coincide with the date of any major battle to make it clear that the holiday honored all fallen soldiers equally.

Detail from DaVinci's Last Supper.
Some Say the Last Supper May Have Been a Passover Seder.6

In looking at the early Christians’ decision, it’s important to remember that they were not raised celebrating Christmas and Easter, their traditions were Jewish, or Roman, or those of one of the other faiths practiced in the Roman province of Judea in the first century. They obviously felt the connection with the date of Passover was important. In places where Christians were persecuted and had to practice their faith in secret, there may also have been the practical consideration that holiday gatherings would be less conspicuous if they occurred at the same time Jewish people were celebrating Passover.

Relevance of the Full Moon. Some early Christians relied on Jewish authorities to determine the date of Passover and, thereby Easter. However, as time went on and the religions drifted further apart, some Christians complained about inaccuracies in the Jewish calculations. The problem was how to determine the date without specific reliance on Jewish authorities or the date of Passover.

Passover begins on the 15th day of the Jewish month of Nisan. Jewish months begin with a New Moon, so given the 29.5 day lunar cycle, the 15th day typically has a Full Moon. Thus, tying Easter to the Full Moon puts it at the correct time in the lunar month without explicit reference to Jewish practice.

Importance of the First Day of Spring. Passover and Easter are intended to be Spring holidays, but because a lunar calendar is based on 29 and 30-day lunar months, a 12-month year is only 354 days. To keep the months in line with the seasons, the Jewish calendar adds a thirteenth month every few years.

Tying the full moon to the first day of Spring achieves the same result for Easter, again without explicit reference to the Jewish calendar.

The First Council of Nicaea.
First Council of Nicaea Established Many of the Rules for the Date of Easter.7

Why Easter Is Always on a Sunday. It wasn’t always. Passover can begin on any day of the week and some early Christians did the same for Easter. Their practice even had a name, Quatrodecimanism, which means 14th day.

Others wanted their observance to be consistent with the Bible’s account that the crucifixion took place on a Friday and the resurrection was two days later, so they made Good Friday a Friday holiday and celebrated Easter the following Sunday. This was the practice that eventually prevailed.

Why Church and Astronomical Events Are Sometimes on Different Days. Timing for the phases of the Moon and beginning of Spring vary by year and location. For example, in the Northern hemisphere, the Vernal Equinox can occur on March 19, 20 or 21. This variability may not be a big deal for a local congregation that can organize a small observance on short notice, but it is unworkable for a global institution that needs to plan and coordinate complex events well in advance.

Thus, when the First Council of Nicaea met in 325 A.D. to resolve matters regarding the date of Easter, it decreed that for Church purposes the Vernal Equinox would be fixed at March 21. That happened to be its astronomical date that year.

To achieve similar predictability for the phases of the Moon, churches treat the 14th day of the lunar month as the time of the full Moon, even though the astronomical event can deviate by as much as two days. The date is sometimes called the Ecclesiastical Full Moon.

Summer solstice meridian line from Saint Supilce in Paris.
Meridian line at the Church of Saint-Sulpice in Paris.8

The Importance of Getting It Right. Development of the the calendar we use today was the result of concern in the sixteenth century that Easter was not being celebrated at the proper time.

Careful observers noticed that over time the Julian calendar was falling out of step with the seasons and the Vernal Equinox seemed to be drifting away from March 21. To correct the shift, Pope Gregory XIII issued an edict revising the calculation of leap years to the system we use today. Churches also were built so that sunlight would touch architectural meridian lines on specified days, providing built-in celestial clocks to verify that the calendar remained in sync with the cosmos.9

* * * * *

The complexities surrounding the date of Easter illustrate how so much of our world derives from long-forgotten considerations of history, religion and science, often seasoned with a dash of “that’s the way we’ve always done it.” But it’s important to remember that no matter how mysterious choices made two millennia ago may seem to us, they likely made perfect sense to people at the time.

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  1. See, Calculating the Easter Date at timeanddate.com.
  2. This period is referred to as a synodic month. The actual length varies between 29.18 and 29.93 days because the orbits of the Earth and Moon are elliptical, not circular. See Wikipedia article on the Lunar Month.
  3. See dateandtime.com, phases of the Moon in Boston for 2018 and 2019.
  4. Stained glass window from the Church of Saint-Sauveur in Plancoët, France. Photo (May 1, 2017) by Wikipedia user Emeltet available on Wikimedia Commons.
  5. Information for this article comes from: First Council of Nicaea on Wikipedia; Ralph Orr, The Passover-Easter-Quartodeciman Controversy on the Grace Communion International web site; and Calculating the Easter Date on timeanddate.com; and The Date of Easter at the U.S. Naval Observatory’s web site.
  6. Detail from Leonardo da Vinci’s fresco, The Last Supper (1495–98), at the Church of Holy Mary of Grace in Milan, Italy. Image from Wikimedia Commons.
  7. Fresco (1590) from the Sistine Chapel depicting the First Council of Nicaea. Image from Wikimedia Commons.
  8. Meridian marker for the Summer Solstice at the Church of Saint-Sulpice in Paris. Photo (2007) by Wikipedia user PHGCOM on Wikimedia Commons.
  9. For more information about meridian lines, see, Geoff Manaugh, Why Catholics Built Secret Astronomical Features Into Churches to Help Save Souls (Nov. 15, 2016) at Atlas Obscura web site.

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Updated April 1, 2019.