To Christians all over the world, Easter Sunday marks the end of Holy Week. It is observed as a day of celebration of Jesus’ resurrection. Yet, as Christians, many of us never questioned the dates we were told to observe Holy Week and Easter. Why does Easter fall on different days each year? How are the dates of the Holy Week and of Easter Sunday calculated? According to articles online, the reason may be found in the way the Hebrews count the days in their calendar.
In olden times, people count the days and months based on the cycles of the moon. They lived in an agricultural society where seasons are used to celebrate the feasts. Since Jesus was believed to have risen on a day so near the spring feasts, it was determined that Easter would always be calculated as the first Sunday after the first full moon after the vernal equinox.
From Wikipedia, we learn that an equinox happens twice in a year, when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, with the Sun being vertically above a point on the Equator. It is named according to the location of the Sun in the celestial sphere. In relation to Easter, the event is approximately set right after the vernal equinox.
How the date of Easter is calculated
According to the Bible, Jesus was crucified immediately before the Jewish Passover, which is a celebration of the Exodus from Egypt under Moses. The celebration of Passover starts on the 14th or 15th day of the (spring) month of Nisan.
Because the Jewish months start when the moon is new, the 14th or 15th day of the Spring month must be immediately after a full moon. It was, therefore, decided to make Easter Sunday the first Sunday after the first full moon after vernal equinox. Or more precisely: Easter Sunday is the first Sunday after the “official” full moon on or after the “official” vernal equinox.
According to the Christian Calendar, the official vernal equinox is always the 21st of March. The official full moon may differ from the real full moon by one or two days. The full moon that precedes Easter is called the Paschal full moon.
Does the Easter date repeats itself every year?
No, the Easter date is neither repeated every year or repeated in a century. There is a sequence of Easter dates, however, that follow the 19 year cycle. This sequence repeats itself every 532 years in the Julian calendar. Amazingly, the number 532 is a product of two significant numbers: 19 (the Metonic cycle or the cycle of the Golden Number) and 28 (the Solar cycle).
How will you know the date of next year’s Easter?
Suppose you know the Easter date of the current year, can you easily find the Easter date in the next year? No, but you can make a qualified guess. According to the Calendar of the Ages at WebExhibits.org, you can guess the date by using the following approximation:
“If Easter Sunday in the current year falls on day X and the next year is not a leap year, Easter Sunday of next year will fall on one of the following days: X-15, X-8, X+13 (rare), or X+20. If Easter Sunday in the current year falls on day X and the next year is a leap year, Easter Sunday of next year will fall on one of the following days: X-16, X-9, X+12 (extremely rare), or X+19. (The jump X+12 occurs only once in the period 1800-2200, namely when going from 2075 to 2076.) If you combine this knowledge with the fact that Easter Sunday never falls before 22 March and never falls after 25 April, you can narrow the possibilities down to two or three dates.”