It is confusing to many people that there are two dates used to celebrate Easter. In fact, the calendar is rather complicated, and without all the scientific instruments we have today it is not a simple matter to work out how long a year is so that it can be divided up usefully. We can work out the cycle of day and night, but even the length of days and nights changed every day so that the day got longer and the night shorter until the longest day of the year on June 21st in our calendar, and then the day began to get shorter and the night longer until the shortest day of the year on December 21st. In between there were two times when the day and night were the same length, and these were called the Spring Equinox and the Autumn Equinox.
For the purpose of working out the date of Easter the Spring Equinox is important, and it takes place on March 20th/21st. If we set a timer on that date when the sun first rose in the sky and measured how long it took until it set, then it would be 12 hours. And if we set a timer when it set and measured when it rose again, then it would be 12 hours. So, the day and night are equal in length, and we use the term Equinox to describe this. It is a fact of the universe. It is not a matter of any religion. We could actually do this experiment of timing the day and night and we would discover ourselves that on March 20th/21st this is what happened ever year.
How do we now when this is going to happen? It is because we have a calendar which has 365 days in it with leap years every now and again. But in the ancient times it was not always clear how long the year should be. Many societies use the moon as a means of calculating time because it appeared in the same shape, a full moon, or a new moon for instance, every 30 days or so. But having 12 cycles of the moon made only 360 days, and the ancient people realised that over a few years things started shifting out of phase with the universe. The shortest day, perhaps a time of lighting fires in the Winter to encourage the light to return to the world, happened at the wrong time, because after 360 days from the last festival it wasn’t the shortest day at all, that was going to happen in 5 more days time, and after two years the shortest day was now 10 days after the festival. It became clear that the year was not 360 days long, but that the universe was running on a slightly longer cycle.
So the early people realised that it was about 365 or 366 days between the things in the skies happening again, the shortest day, the longest day, and the days where the day and night were the same. Many people coped with these additional necessary days by adding extra days in a small month or to different months of the year. The Egyptian calendar followed this model and has 12 months of 30 days, and a small month to accommodate the extra days necessary. But even with this correction, things were not quite accurate and the days still drifted away from things happening in the universe. This is because the year, we now know, is not 365 days long, but is about 365.24 days long. So if the year was 365 days long then every four years or so the festivals and activities of people based on the calendar would be 1 whole day wrong according to the universe. The shortest day in the Winter, returning to that example, would be 1 day after the festival, and this difference would increase about 1 day every four years. But it clearly was not 366 days long either, because this would mean that festivals were taking place after the universe. So the shortest day would happen one day before the festival, and the two days and so on.
It was realised that what was necessary was an additional day which soaked up this difference. In some societies this was added when it seemed required. Either the religious or civil authorities would announce extra days to bring the calendar back into line with the universe, so that, for instance, a festival on the shortest day actually took place on the shortest day.
In Rome, this process of adding extra days belonged to the civil authorities, and they realised that they could retain their governmental positions longer by extending the year, since they were elected for a year of authority and influence. This led to corruption of the calendar, since they could keep putting off the end of one year by adding more days. Julius Caesar acted to correct this situation and also to bring about a rule for the year that did not depend on the personal intervention of civil authorities.
In a simple explanation, he introduced a fixed leap year every four years which would add one extra day, so that the length of the year kept close track with the actual universe. This meant that every time the festivals of the year might start drifting away from the reality of the universe, the leap day being added would bring it back to connect the festivals, such as for the shortest day, with the actual shortest day in reality. At the time he introduced his rules the Roman year had to be changed all at once because the calendar had drifted so far from reality that it was a about a whole month disconnected from reality. This would be like celebrating the shortest day of the year in the middle of January instead of December 21st when it actually takes place.
His intention was for his calendar calculation to match the reality of the universe as closely as possible. By introducing a year of a length of 365.25 days he was very close. But the year is not 365.25 days in length, and his improvement had a built-in error. Over the length of even two lifetimes this difference was not so noticeable, but it became apparent very early. Here is the problem. If a leap day is added every 4 years then over the course of 400 years 3 more days are added than are required. And over the course of 2000 years, 13 days have been added more than were required.
This means that a Roman citizen, using the much improved calculation developed by Julius Caesar, if he celebrated the shortest day of the year on his calendar would be doing so 13 days later than reality, because leap days had been added when they were not necessary over the centuries. This also means that the Spring Equinox according to this Roman citizen’s diary, would not happen according to the reality of the universe but would also happen 13 days later. Every year would be 13 days out of synchronisation with the reality of the universe because the pagan Roman Emperor Julius Caesar, when introducing an improvement to the pagan Roman calendar of his time, left a small adjustment which we was not aware of, but which would create a disconnect with reality over the course of time.
The Egyptian calendar had its own months of 30 days, and an additional small month to provide the extra days needed each year. This calendar also adopted the same method of calculation as Julius Caesar. It provided 365 days every year, and 366 every fourth year, and so this also introduced the same error over the longer scale.
When the Christian Church came to develop a cycle of festivals and seasons it adopted the civil calendar in use around the Empire. This was not because Julius Caesar was a saint, but because his calendar worked. The Spring Equinox, when the day and night were equal, happened on the right time every year, at least for the first centuries. The festivals of the Lord Jesus, of the saints and martyrs, were added, and these happened at the same time each year according to the calendar, and in synchronisation with the universe.
The calculation of Easter was not without some controversy, but it was accepted from the beginning of the Church that it should be a moveable feast, in connection with the Passover at which it took place, rather than a fixed date. The rule became that Easter should be celebrated on the first Sunday after the first Full Moon after the Spring Equinox. There was some possibility of disagreement in this calculation. The actual date of the Spring Equinox varied slightly between March 19th and March 22nd, and it was not always agreed when the Full Moon occurred. This meant that at an early period the Church developed its own rules based on an idealised situation where there was a fixed cycle of the phases of the moon, and where the Spring Equinox was assumed to take place on March 21st. This brought coherence to the celebration of Easter in most places, although there were still years where different churches, using the same rules, came out with different results.
This is not the reason why we celebrate two dates for Easter. Even in the early centuries it became clear that the Spring Equinox was not at the time expected, because slowly more and more days were being added to the calendar because there were three too many leap years in every 400 years. This is not a new observation. Eventually, by the 16th century, the calendar created by Julius Caesar was 10 days different to the actual universe. The Spring Equinox took place, but the calendar now said March 21st was ten days later. The connection between the real world and the calendar on a piece of paper had been lost, even though it was a connection that Julius Caesar intended, and that the early Church relied on.
The Roman authorities realised that by simply changing the rule about leap years and bringing the calendar back into line once and for all, as Julius Caesar had done, then the paper calendar would match the actual reality of the universe again and would not drift out of synchronisation. All that was required was that instead of having a leap year every four years, instead, on the occasions when it was a century – 1600, 1700, 1800 etc – there would only be a leap year if the year could be divide by 400. Therefore 2000 was a leap year, but 2100 will not be. There was also a need to change the paper calendar once so that March 21st, for instance, was actually at the time of the Spring Equinox. The Romans achieved this by simply jumping from the Julian calendar day Thursday, 4 October 1582 to the first day of the Gregorian calendar, Friday, 15 October 1582. Friday followed Thursday as expected, but by correcting those 10 days error which had accumulated the calendar now matched that which had been created by Julius Caesar and used by the early Christians.
There was a limited adoption of this correction. The calendar stayed exactly the same of course. All of the feasts were on the same date, but the date now matched the actual reality of the universe, especially for the calculation of Easter. This correction was adopted by Rome in 1582, and at the same time by Spain, Portugal, France, Poland, Italy, Catholic Low Countries, and Luxemburg. Great Britain did not adopt it until 1752, by which time the error was 11 days. Now the error is 13 days.
The Eastern Churches did not adopt this correction, for a variety of reasons. In the 20th century some of the Orthodox Churches began to adopt the correction, but continued to use the old method, with its built-in astronomical error, to calculate Easter, for the sake of unity. Some of the Orthodox Churches had adopted the correction even for the calculation of Easter, such as the Orthodox Church of Finland, the Armenian Orthodox Church, and the Indian Orthodox Church.
The difference in date essentially comes down to whether or not the things we say are happening according to a paper calendar should actually be happening in the universe around us. This especially concerns the Spring Equinox which is used for calculating Easter. The early church expected that when they calculated Easter it would be in relation to the actual state of the universe, and so the Spring Equinox being used would certainly be approximately happening at that time. This is why the correction was made. It did not change a single feast or fast, that is not what the correction meant, and to say the calendar is in error is not to speak of the cycle of feast and fasts at all, but it is to say that what we are doing has lost a connection with the reality of the universe and this is not what the early Church expected or intended.
If a calculation is made for Easter when it is assumed that the Spring Equinox will be 13 days later than it actually takes place, then Easter will often be calculated on a different day than for those who do assume it to be at the real and actual time of the Spring Equinox. This is not a matter of loyalty to the Tradition, since the early Church used a calendar which they expected to be accurate and which they intended to use accurately. The correction of the pagan calendar of Julius Caesar means that an extra day is added by mistake every 3000 years instead of every 130 years. It also means that we are able to follow the actual calendar of the early Church and make a calculation for Easter as they intended, in relation to the reality of the actual universe.
This does not mean that either Easter is right or wrong. It is never the celebration of a feast which is wrong. But if we are concerned with an expression of Christian unity then continuing to using the same calendar, but correcting one obvious error in its use, is a means of doing so without changing any aspect of our Tradition, but rather bringing us back into synchronisation with the universe as God has established it, and the calendar as the first Christians employed it. This is not a theological issue, nor is it a matter of being traditional. It is a practical issue that has many benefits and no disadvantages. May we not be afraid to correct such practical matters for the sake of better expressing our ancient faith in modern times.