Why Easter Hops Around on Different Days

By Jacqueline G. Goodwin, Ed.D.

In 2018, Easter was on April 1. In 2017, it was on April 16. And in 2016, it was on March 27. Unlike other holidays that have set dates, Easter falls on a different date each year. Why is this so?

According to the Christian Gospels, Jesus was crucified while the Jews were celebrating the week-long festival of Passover.  The Gospel of Mark indicates that Jesus’s last supper was a Seder, the feast held on the first evening of Passover.  The other three Gospels indicate that the last supper took place when Passover was already underway. For this reason, the early Christians celebrated Easter at the same time that the Jews celebrated Passover. n 2018, Easter was on April 1. In 2017, it was on April 16. And in 2016, it was on March 27. Unlike other holidays that have set dates, Easter falls on a different date each year. Why is this so?

Passover starts on a specific date of the Jewish calendar which is the 15th of Nisan. It is a hybrid lunar-solar calendar, and every month begins on the night of the full moon. This means that inserting leap days as the Gregorian calendar does to keep the calendar aligned with the solar year is impossible. Instead, the Jewish calendar inserts an entire leap month every two or three years. The dates of Passover, therefore, vary from one year to the next in the modern Gregorian calendar, based on the ancient Roman calendar, while always falling in March or April. This is essentially the same reason why the Chinese New Year, fixed in the semi-lunar Chinese calendar, varies in the Western calendar, while always falling in January or February.

Christians eventually decided to adopt their own rule for fixing the date of Easter.  The First Council of Nicaea, held in the year 325, decreed that Easter should be observed on the first Sunday after the first full moon of the spring. The reason why this rule combines a lunar criterion or full moon and a solar one is to keep up with the semi-lunar Hebrew calendar, without having to use it directly.  In fact, Good Friday, the Friday before Easter, usually coincides with the start of Passover, though not always exactly.

For the purposes of finding the date of Easter, both the start of the spring and the time of the full moon are determined by a simplified calendar rule, rather than by the actual astronomical events or the vernal equinox and the opposition of the moon and the sun.  This not only makes the calculation easier, it also avoids having the date of Easter depend on the geographic location of the observer.

According to the rule adopted at Nicaea, spring is taken to start on March 21, while the date of the “ecclesiastical full moon” is taken from the “Metonic cycle” of exactly 235 full moons in 19 years.  Easter is the first Sunday after the ecclesiastical full moon that occurs on or immediately after March 21.  Thus, the earliest that Easter can occur is on March 22, if the full Moon is on Saturday, March 21.  The latest that it can occur is April 25, if the full moon is on Sunday, March 20. The dates of Easter repeat after a cycle of 5,700,000 years.

Thus, if the first full moon of spring occurs on Wednesday, March 20, it would stand to reason that Easter 2019 should take place on the following Sunday, March 24. But it doesn’t, because of two ecclesiastical rules – rules that pertain to the Christian Church and its clergy.

The Golden Number vs. Astronomy

First, there is the dating of the full moon. Astronomers can tell us precisely to the exact minute when the moon will arrive opposite the Sun that will brand it as a “full” moon. However, the Church follows its own methodologies in determining when the moon turns full. One important factor is something called the “Golden Number.”

It is a rather arcane series of computations that in the end provides a date for Easter. Of course, on occasion, the date for the full moon does not exactly line up with the date that is provided by astronomy.

Second—and this is the primary stumbling block for this year—is that from the ecclesiastical perspective, the first day of spring falls on March 21. Historians say that March 21 was selected as the ecclesiastical vernal equinox because the Church of Alexandria, whose staff were reputed to have astronomical expertise, said that March 21 was the date of the equinox in 325 AD. But the date of spring differs in astronomy. In our lifetimes, for the longitudes of North America and for Europe, spring won’t arrive on March 21 until the year 2102.

Because the March full moon falls on the 20th and not the 21st, it is not recognized as the Paschal Moon by the Church. As a result, we must wait until the next full moon, on April 19. That day is also Good Friday and that evening is the first night of the Jewish feast of Passover. Finally, on Sunday, April 21, we will celebrate Easter Sunday. This is four days from the last possible date for Easter.

Incidentally, finding the date of Easter for a given year in one’s head has long been regarded as one of the greatest feats that a “human calculator” can accomplish. Neurologist and popular science writer Oliver Sacks mentions this feat in his book, The Man Who Mistook His Wife for a Hat. Assuming you are not so inclined to determine when Easter falls in a certain year in your head,  if you want to know the date of future Easter Sundays, you can always refer to Wikipedia’s table that gives the dates of future Easter Sundays all the way to the year 2039.