Easter (Resurrection Sunday) is April 21. At least it is this year. Last year it was on April 1, and next year it will be on April 12. And these are the dates for Resurrection Sunday in the West. In the East (Orthodox Churches), Resurrection Sunday is observed a week later in these three years. That’s the usual circumstance, but some years the dates for Resurrection Sunday in the West and East coincide, while in other years Resurrection Sunday is observed more than a month later in the East than in the West. What is going on? And why does the date of Resurrection Sunday bounce around on our calendars? The answers to those questions are found in the algorithms for defining the date of Resurrection Sunday.
Why does the date of Resurrection Sunday bounce around on our calendars?
We celebrate Easter to observe the resurrection of Jesus Christ. Many people know that Jesus rose from the grave early on the first day of the week (Matthew 28:1-7; Mark 16:1-11; Luke 24:1-9; John 20:1),1 so that takes care of the Sunday part. But what was the time of year? We know from the gospels that Jesus was crucified at the time of Passover (Matthew 26:17-18; Mark 14:12-16; Luke 22:7-13; John 12:1). Passover was the commemoration of the Israelites’ liberation from bondage in Egypt (Exodus 12:1-28; 23:15; Leviticus 23:5-8). The Exodus occurred on the 15th day of the first month in spring (Aviv, or Nisan), so this is how the date of Passover has been observed ever since.
Calculating Months Based on the Sun and/or the Moon
However, the Hebrews kept a very different calendar from our calendar today. Our modern calendar is a solar calendar, in which the length of the year is defined by the tropical year, the orbital period with respect to the vernal equinox. This keeps our calendar in sync with the seasons. However, our months bear no resemblance to the true month, the orbital period of the moon around the earth. Consequently, the phases of the moon drift progressively earlier on our calendar. Some ancient calendars (and even the Islamic calendar today) were strictly lunar calendars. Lunar calendars keep the moon’s phases in synch with the months, but lunar calendars drift about ten days earlier with respect to the seasons each year.
In contrast, a lunisolar calendar preserves the true lunar months and makes allowances to stay synchronized with the year. The ancient (and modern) Hebrew calendar is a lunisolar calendar. To keep the strictly lunar months, the months on the Hebrew calendar generally alternate between 29 and 30 days. After 12 months, about 354 days have elapsed, roughly ten days short of a full year. To fix this mismatch with the year, a 13th month is added about every third year. Since the civil calendar adopted at Sinai begins near the vernal equinox (the month of Aviv), this intercalary month, when necessary, is inserted before Aviv and the beginning of the new year. Passover, being the 15th day of Aviv, always coincides with a full moon. Therefore, to make Resurrection Sunday coincide with the date of Passover in some manner, Easter is defined as the first Sunday after the first full moon after the vernal equinox.
Easter is defined as the first Sunday after the first full moon after the vernal equinox.
To check this out, let’s examine the date of Resurrection Sunday in 2018. The vernal equinox in 2018 was Tuesday, March 20 at 21:58 Greenwich Mean Time (GMT). The full moon that month was Saturday, March 31. Therefore, the following day, Sunday, April 1, was Easter. Incidentally, Passover in 2018 was March 31 (beginning at sundown the night before). Consequently, this seems like a good match between Passover and Resurrection Sunday. There usually is a good match, but not always, for sometimes Passover and Easter are a month apart.
Julian and Gregorian Calendars
So far, so good, but why was Eastern Resurrection Sunday on April 8 in 2018? This has to do with another quirk. The ancient Romans originally had a lunisolar calendar, but Julius Caesar replaced that calendar with a solar calendar in 45 BC. The Julian calendar, as this calendar is called, was the calendar used throughout Europe and the rest of the old Roman Empire until a few centuries ago. In 1582, Pope Gregory XIII instituted the Gregorian calendar to correct a slight error in the Julian calendar. This is the calendar that most of the world uses today. Part of that reform was to delete ten days from the calendar in October 1582 (October 4 was immediately followed by October 15 that year). While western Europe largely came to adopt the new Gregorian calendar, the Orthodox Church did not. The formula for determining the date of Easter used by the Eastern Church is based on the Julian Calendar rather than the Gregorian calendar, which results in the date often being a week later than in the West, and sometimes more than a month later.
When Is the Vernal Equinox?
However, this year poses an interesting problem. The vernal equinox in 2019 was on Wednesday, March 20, at 21:58 GMT. And full moon was on Thursday, March 21, at 1:53 GMT, nearly four hours after the vernal equinox. So why wasn’t Resurrection Sunday, March 23 rather than April 21 this year? The answer lies in how the date of the vernal equinox is defined in the algorithm used for determining the date of Easter. That formula was agreed upon at the Council of Nicaea in AD 325. At that time, the date of the vernal equinox was March 21, so that became the definition of the vernal equinox. However, we can more precisely determine the time of the vernal equinox today than people could 17 centuries ago. Therefore, the usual definition of the date of Easter that I gave above isn’t as precise as it ought to be. Perhaps we ought to say that Resurrection Sunday is the first Sunday after the first full moon following March 21 (which, by definition, is the vernal equinox).