Why Is Easter When It Is?

This year we celebrate Easter day on 16 April, three whole weeks later than last year. At its earliest, Easter can fall on 22 March, although it will be 250 years before this happens next. The latest date for Easter is 25 April, which will next occur 21 years from now, and in some countries the date falls another thirteen days later still.

So why is there such a huge variation in Easter’s date?

It depends on the moon and the sun and the way calendars are calculated, and it’s complicated! But it’s fascinating too, so stay with us!
We celebrate Easter to remember the Resurrection of Christ which, according to the Bible, took place around the time of the Jewish Passover.
At the time of Jesus Christ, people were using two different methods to mark the passing of time: one based on the moon and the other on the sun. The Jews had been using the lunar calendar for 2,000 years, while the solar calendar was the basis for the Julian calendar, introduced to the Roman Empire by Julius Caesar in 45 BC. (You can read all about this in our earlier blog post on the names of the months if you are interested).
A year of twelve lunar cycles is slightly shorter than a solar year so it was almost impossible to translate a date from one calendar to the other. Even if the precise date of the Resurrection had been recorded, which it wasn’t, it would have been difficult to know when to celebrate the anniversary. So with no fixed date and two different calendars in use, it’s not surprising that for the first few centuries after the life of Christ, people celebrated Easter on many different dates.

One date based on two different calendars

As Christianity spread through the Roman Empire, church leaders realised the need for a single date for the religious festival and they discussed the issue at a meeting in Nicea (in present day Turkey) in the year 325. With Eastern churches still following the lunar calendar and Western churches using the solar calendar, it became clear that the chosen date would have to be based on both calendars.
Eventually they agreed that Easter should fall on the first Sunday following the first full moon following the spring equinox. This meant that the lunar calendar was represented by the full moon and the solar calendar was used for the equinox (the day when night and day are exactly the same length, and the moment when the sun crosses the celestial equator). The actual date of the spring equinox can vary by three days, and so, to make it easier to plan ahead, the church leaders agreed to recognise 20 March as the notional date for the equinox, regardless of when the astronomical equinox actually took place.
They also agreed one exception to the rule. In any year in which Passover should fall on the same day as Easter, Easter would move one week later so that the two festivals did not coincide.

A flaw in the Julian calendar

This formula worked fairly well for the next 1,000 years or so. But in the mid-1200s, an English Friar called Roger Bacon noted that the date of Easter was occurring later and later in the spring.
A flaw in the way the Julian calendar was calculated meant that the calendar had too many leap days and was moving ahead by 11 minutes each year. Over 1,000 years this had accumulated into nine days. This meant that 20 March was gradually drifting ahead and moving further and further from the actual astronomical equinox.

Correcting the drift

In the mid-1500s Pope Gregory established the new improved Gregorian calendar which removed eight of the 250 leap days every 1,000 years. This solved the problem for the future but there was still the question of the days which had already drifted since the Julian calendar first began, and which had now accumulated into ten days. At the same time as introducing the new calendar, Pope Gregory corrected the date by removing those ten days, bringing 20 March back to the actual time of the spring equinox, which made the Easter calculations true again.

Why is Easter thirteen days later in some countries?

In some parts of the world Easter is celebrated thirteen days later than in the UK. That’s because some parts of the Eastern Orthodox Church still use the old Julian calendar to decide the dates of some religious festivals. As the years go by, the Julian calendar drifts still further ahead and by Easter 2100 the difference in dates will extend to fourteen days.
All this, and we haven’t even mentioned the symbols of eggs; including the word ‘Easter’ in any term about ‘Egg Hunts’ or for that matter Easter Bunnies. Maybe for another time…………