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There are three possible Easter dates depending upon the year and your cultural and religious persuasion. Most Easter information elsewhere on the Web is written with the author's own circumstances foremost. This article gives just the alternatives.
Definition of Easter Sunday Date
Here is a table showing all Paschal Full Moon Dates:
Sadly, many definitions of Easter on the Internet and in Encyclopaedias and Almanacs are misleading, ambiguous and just plain wrong! This is obvious with the application of plain commonsense. A typical wrong definition is:
This is wrong!
Vernal means springtime, and countries in the Southern hemisphere have opposite seasons to those in the Northern hemisphere. Of course, Easter is not celebrated in September in the southern hemisphere! So the reference is really to the March Equinox, but even that is equally wrong in this definition, but for different reasons (see below).
Also, I think that almost everyone reading this would assume that "full moon" refers to an astronomical full moon date. An astronomical full moon (AFM) occurs at one instant in time, and therefore occurs on 2 dates around the world (at any instant it is midnight somewhere in the world, with different dates for locales on either "side" of midnight). Again, countries do not celebrate different Easter dates based upon their own full moon dates!
And it gets worse!
Some people have tried to simplify this concept by using one "after" with the date to March 21, so it reads "the full moon after March 21". This logic is seriously flawed!
It is further compounded with phrases like "from March 21". This is unclear whether it means "from and including March 21" or "after March 21". One bizarre encyclopaedia definition gets it so wrong that it concludes that Easter Sunday can never fall on March 22! Absolute rubbish!
How are full moons related to Easter?
PFM dates are found in a table (see above). There is a table of 19 dates for the Julian calendar, and several 19-dates tables for the Gregorian calendar.
From 1583, Astronomical and Paschal full moon dates never differ by more than 3 dates, even taking into account the 2-date AFMs (see above). For example, an April 11 Easter Sunday could result from:
For most Easter Sundays, the nearest astronomical full moon date can be anything from 10 days earlier (over a week before) to 2 days later (on the Tuesday after Easter).
How is the equinox related to Easter?
What can I do when I see a wrong definition?
This is right!
Alternatively, you can refer them to this webpage, or to the most authoritative article on Easter dating I have seen at the Astronomical Society of SA for a complete explanation.
This definition is correct, and can be easily proved by checking historic Christian definitions and Easter Sunday dates.
Further, this definition is accurate for each of the 3 different implementations of Easter Sunday:
To Summarise ...
Some related facts ...
Here is a picture showing how these different dates relate to each other:
The Gregorian calendar has gradually replaced the Julian calendar over a period of a few hundred years from October 1582 to re-align dates with the changing motions of the earth and moon in relation to the sun.
Christians of Western denominations have generally adopted a revised Easter Sunday calculation based upon the Gregorian calendar, which more closely aligns PFM dates with astronomical full moon dates. This revised calculation has often been used from the commencement of the Gregorian calendar. In other cases it has been applied at a later date, or not at all (eg Orthodox churches).
1583 was the first year Easter occurred with the revised calculation in the new Gregorian calendar. Devised predominantly by Lilius and Clavius, they were introduced in October 1582 by Pope Gregory XIII.
1753 was the first year Britain and its colonies (at that time) used both the Gregorian calendar and the revised calculation for Easter.
Here's a page showing history of the major events that shaped the calendar and Easter dating methods.
The Three Calculations
The Gregorian calendar has gradually been adopted world wide from October 1582. The last known use of the Julian calendar was by Greece in 1923, so method 1 applies only historically.
Either at the time of their calendar change or at a later date, some (but not all) regions have used the revised Easter date calculation based on the Gregorian calendar. The current Gregorian calendar is valid until at least 4099 AD.
At the adoption of the Gregorian calendar, most Western churches moved from method 1 to method 3, while Orthodox churches moved from method 1 to method 2.
A Country Guide
Here is a guide on which method to use.
It is important to check the history of the region in question to find the correct date of their change from Julian to Gregorian calendar, and if applicable, their change from the original to the revised Easter Sunday date calculation.
When you know which method to use, click this link for an algorithm to calculate any Easter date
Here are some other Web pages on Easter Date Calculation.