How Easter dates are calculated

this year is almost as late as it gets.
In fact, there are not that many people alive who have seen such a late Easter, although a reasonable group will see another one.
The only occasion in the 20th century when Easter was in the last two permitted days was in 1943, when it actually fell on April 25.
The last time it was on April 24 was in 1859.
There will be a couple more very late Easters this century. In 2038 it will fall on April 25 and in 2095 it will fall again on April 24.
Calculating the date of Easter has been a subject of some contention for Christians right back into the early Church.
It is not only the biggest festival in the Christian church calendar but also the earliest: the ressurection of Christ being the central event in history for Christians.
It was originally based purely on the Jewish calendar, since Jesus was cruxified the day after the Passover. But in 325, at the Council of Nicea, Christians agreed that they should have their own way of doing the calculation.
There were problems with the Jewish calendar, not the least being that not all Jews could agree on what the correct dates for the Passover were so different Christian churches tended to follow their local Jewish community’s calculation and everyone wanted all Christian Easters to be on the same Sunday.
While the rough and ready rule can be thought of as the first Sunday after the first full moon on or after the March 21 equinox, it is not that simple.
The canonical rule is that Easter day is the first Sunday after the 14th day of the lunar month (the nominal full moon) that falls on or after March 21 (nominally the day of the vernal equinox).
For determining the feast, Christian churches settled on a method to define a reckoned “ecclesiastical” full moon, rather than observations of the true Moon.
In modern language, this definition is best described as: Easter is the Sunday following the Paschal Full Moon date. The Paschal Full Moon date is the Ecclesiastical Full Moon date following March 20.
But even this involves pages of calculation and certain assumptions (the assumptions made by the Patriarch of Alexendria eventually becoming he norm if only because he had the best mathematicians and astronomers in the later Roman Empire) although over the centuries alograthims, complex formulas, have been developed to simplify the process of fitting a lunar calendar into a solar calendar.
All churches used the same date (with only a minor row involving the British churches in the fifth century) until the calendar reforms of Pope Gregory XIII in 1582.
This led to March 21 falling on different days in the Catholic world and the rest of Christendom although the Protestant world came round over the next hundred years. New formulas were created to take into account the lack of leap years in those century years not divisible by 400.
Most Eastern Orthodox Christians calculate the fixed date of March 21 according to the Julian calendar and use an ecclesiastical full moon that occurs four to five days later than the Western ecclesiastical full moon. But every four years all Christians, Western and Eastern, celebrate Easter on the same day. 2011 is one of those years.