This was first published on the WWW 1995 June 25 by Marcos J. Montes.
This version was created 2001 July 31 by Marcos J. Montes.
Reviewed in Rettig on
Reference Newsletter for October, 1995.
One of the 500 sites listed in Catholicism
on the Web.
Named a MAGELLAN 3 STAR SITE on
1996 January 20.
Named a POINT TOP 5% SITE on
1996 February 27.
1997 July 5
#2 in Calendars Topic on 1999 October 1.
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Here is a list of Orthodox Easter dates listed in the Julian Calendar or the
Gregorian Calendar, 1875-2124.
Here is a list of Western Easter dates AD
Here is a list of years with the same Julian date or Gregorian date of Orthodox Easter, AD
Here is a list of years with the same date of
Western Easter, AD 1875-2124.
Here is a Table of the frequency of the
difference between the dates of Orthodox and Western Easter, AD 1583 to
This program calculates:
I have summarized some information on the Orthodox
and an algorithm by Gauss to calculate the date of the Orthodox Easter.
- Easter Sunday in both
the Western (Catholic, Anglican, and Protestant) Christian, and
in the Eastern (Orthodox) Christian traditions;
- The movable Feasts associated with Easter in the Western
and Eastern traditions;
- Many Feasts in the Catholic calendar, usually celebrations of
events in the lives of Jesus or Mary. (These Feasts may or may
not be celebrated by other Christian denominations.)
- For years before 1583, only the dates of Christmas and
Easter are calculated. (They are the same for both traditions pre-1583.)
My recent research (email and bookwork)
has shown that
the above method for calculating Easter prior to AD1583 was not in wide usage
until the mid-eighth century, although it had been adopted by the Church in the
- An important note for historians and people using these
dates for research: Even though the Gregorian calendar was
use by the Catholic Church and many Catholic areas of Europe in AD 1582
many areas did not adopt the Gregorian Calendar, the new method of determining
Easter, or both, until later. A good review is given in
reference (1). For example, England and its dominions did not
accept the Gregorian Calendar or the new method of determining Easter
until 1752; thus, Easter in England prior to 1753 was determined using
the same algorithm as that of the Orthodox Church.
Prior to AD325, churches in different regions celebrated Easter on different
dates, not always on Sundays.
The Council of Nicea (AD 325) clarified this a bit by stating that Easter would
be celebrated on Sundays. Still a number of methods were used until a method
defined by Dionyisius Exiguus was adopted in about AD 532.
This was not widely accepted until it was described and defended by the
Venerable Bede in his De temporum ratione
[Thanks to Jim Morrison
(email@example.com) for the previous four sentences.]
Aloisius Lilius (d. 1576) devised the system that
would become the basis of the Gregorian Calendar, as well as the
tables that would be used to determine the date of Easter.
Christoph Clavius modified the tables slightly, and
was one of the prime defenders of the Gregorian calendar. The tables
used to determine the date of Easter (in the West) since AD 1583 are
these modified tables of Clavius. All algorithms for calculating the
date of Easter since then are based on these tables.
Easter is the Sunday after the Paschal Full Moon. The
Paschal Full Moon may occur from March 21 through April 18, inclusive.
Thus the date of Easter is from March 22 through April 25, inclusive.
The date of the Paschal full moon is determined from tables, and it may
differ from the date of the the actual full moon by up to two days.
This definition, along with tables, etc. may be found in "The
Explanatory Supplement to the Astronomical Ephemeris and American
Ephemeris and Nautical Almanac". This definition that uses tables
instead of actual observations of the full moon is useful and necessary
since the the full moon may occur on different (local, not
UT) dates depending where you are in the world. If the date of Easter
was based on local observations, then it would be possible for
different parts of the world to celebrate Easter on different dates in
the same year.
To further confuse the issue, many countries did not
start using the Gregorian calendar in October 1582, so Easter in those
countries was celebrated at times different than is listed here UNTIL
they began using the Gregorian calendar. And some countries that switched
to the Gregorian calendar used a different
definition of Easter for some time (parts of Germany and Sweden used
tables based on the observations of Tycho Brahe to determine Easter for
many years after the Gregorian calendar was adopted in those locations).
A reasonably comprehensive list of when nations started to use the
Gregorian Calendar may be found in reference (1) and in
the Calendar FAQ.
An interesting upshot of the algorithm is that the
cycle of Easter dates (in the Gregorian Calendar) repeats every
5,700,000 years - and no sooner! (See the Calendar
FAQ for why the period has this particular length.) Using the
algorithms, I have calculated the distribution of the Gregorian Easter
dates over various periods of time. You may view the frequency of the
date of Easter over one complete 5,700,000 year
cycle, or over the first complete 400 year
Gregorian Calendar cycle, or over a more contemporary timespan of
1875 to 2124.
The algorithm used to calculate
the date of Easter in the Western tradition (after 1582) is
from Practical Astronomy with your Calculator by Peter
Duffett-Smith and he got it from "Butcher's Ecclesiastical Calendar"
(1876); apparently the algorithm was first published anonomously in
Nature in 1876. This particular algorithm
uses just integer math. The algorithm is valid for all years
in the Gregorian calendar, that is October 1582 and onwards. Carter's algorithm is a more simple method for
calculating the date of Easter and it is valid only from 1900 until 2099.
Doggett's modification of Oudin's algorithm is
easy to use and is valid
after AD 1583. Mallen's
method is another general, easy to use method.
Some published methods do not for work for all years, and the method
at this link
from the 11th Edition Encyclopedia Brittanica unfortunately
fails in some cases.
There is a useful collection of articles gleaned from the
discussions that contains some history, as well as explanation of the
algorithms used by both the Orthodox and Western churches that has
been collected as an Easter-Date
FAQ; it has both some C code and an amazing Bourne shell script
for calculating the date of Easter.
Finally, there is a
due to Gauss for calculating the date of the Orthodox Easter.
A few other useful algorithms are also listed on my American Secular Holidays Page. This includes
algorithms for determining dates such as "The Second Sunday in May".
Future Validity of All Algorithms of Determining the Date of Easter
There are many reasons to expect that all methods of determining the
date of Easter will not be valid in the far future. The prime physical
reason is that the length of the day is increasing, thus the number of
days in a year is slowly decreasing. The current rate of increase in
the length of the day implies that the Gregorian calendar will need to
neglect a leap year sometime in the 4th or 5th millenium.
A greater likelihood is that some time in the near future the date of
Easter may be fixed to a particular Sunday. At Vatican II, Pope John
XXIII stated that there was nothing wrong with fixing the date of
Easter. And there seems to be broad support in the World Council of
Churches for a fixed celebration of Easter. According to the
Encyclopaedeia Brittanica, the second Sunday in April is
the most favored date. Fixing the date of Easter to a particular
Sunday would still mean that Easter and the Feasts related to it would
be movable, but the movement would be restricted to a span of seven
dates (for example, the second Sunday in April must fall between April
8th-14th). Most of the discussion on this issue appears to have
happened in the 1960's-1970's, but there is a press release from the Aleppo meeting of
the World Council of Churches that
discusses new proposals for fixing the date of Easter for all of
Chrisitianity. The press release is dated 1997 March 24, and the basic
suggestion is to use astronomical measurements of the vernal
equinox and the full moon at the meridian of Jerusalem in order to
determine the date of Easter. The authors of the proposal wanted
this method to be adopted in the year 2001. Currently no Church has
adopted this proposal. Please visit William Morris' New Easter Dates
website for a comparison of Easter dates in the Gregorian and Orthodox
Calendars, along withdates calculated using the Aleppo proposal,
and dates calculated as the Sunday after Passover.
Feasts Related to Easter
Of interest from the reference (1)
(which is a pre-Vatican II source!) are the following dates, and their
relation to Easter.
Days Before Easter Days after Easter
Septuagesima 63 Rogation Sunday 35
Quinquagesima 49 Ascension 39
Ash Wednesday 46 Pentecost 49
Palm Sunday 7 Trinity Sunday 56
Good Friday 2 Corpus Christi 60
Matthew Bear (firstname.lastname@example.org) was the first to inform me
that Septuagesima, and Quinquagesima were in a pre-Lent season
prior to Vatican II (in the Catholic church) or the 1970's (various Protestant
denominations); Rogation Sunday was the Sunday before the Rogation
(prayer, supplication) days before Ascension.
writes that in the Eastern Orthodox perspective, Septuagesima and
Quinquagesima are still celebrated- in fact, the three Sundays of
Pre-Lent before Lent starts have as their Gospel Readings:
the Parable of the Publican and the Pharisee,
the Parable of the Prodigal Son, and Christ's reminder of the Last Judgement.
Celebrations in the Ecclesiastical Calendar
Not Related to Easter
Once we have determined a date of the year and a day of the week,
we can fix each date of the year to a day of the week. While my algorithm
uses Easter to do this, general algorithms exist that allow the determination
of the day of the week for a particular year (see the
Sundays in Advent are determined in the following
method. First, the feast of Christ the King is the Sunday on or after
20 November; the First Sunday of Advent is the Sunday on or after 27 Nov.;
the Second Sunday of Advent is on or after 4 Dec.; the 3rd Sunday of
Advent is on or after 11 Dec.; finally, the 4th Sunday of Advent is
on or after 18 Dec. The day of the week that Christmas
falls on can then be easily determined.
Other Feasts that are listed by the Ecclesiastical Calendar are:
The Solemnity of Mary on 1 January;
Epiphany on 6 January (traditional) or the 2nd Sunday
The Presentation of the Lord on 2 February;
The Annunciation usually on 25 March;
The Transfiguration of the Lord on 6 August;
The Assumption of Mary on 15 August;
The Birth of Virgin Mary on 8 September;
The Celebration of the Holy Cross on 14 September;
The Mass of the Archangels on 29 September;
and All Saints' and All Souls' on
1 November and 2 November, respectively.
I still have limited the determination of these feasts
to dates in the Gregorian Calendar. It is not impossible
to calculate feasts for dates before then - I just have not done it. In
addition, until recently some of the celebrations I list may not have been
standard, defined, or celebrated on the dates currently listed. (In particular,
some of the celebrations may have been celebrated on different dates before
- "The Explanatory Supplement to the Astronomical
and American Ephemeris and Nautical Almanac" (1961, Her Majesty's
Stationery Office [QB8.G82.1962]) has tables for
calculating Easter, as well as the dates various countries/regions adopted the
Gregorian calendar, and other very useful information on calendars and
ephemerides. Many of the relevant sections may be found at Calendars and their
by Claus Tondering
answers many questions concerning calendars, leap years, the Christian, Hebrew,
and Islamic calendars. Check this out if you want to find the algorithm
for making a Hebrew calendar.
- The Easter-Date
FAQ has useful historical and algorithmic information on how the
date of Easter is determined.
- "Practical Astronomy with your Calculator" (2nd Edition, 1981,
Cambridge University Press) by Peter Duffett-Smith has many useful algorithms,
including the calculation of the date of Easter in the Gregorian Calendar
that I use.
- "Butcher's Ecclesiastical Calendar" (1867) is the original source
of the algorithm that I use.
- "The Gregorian Calendar" (1982 May, Scientific American,
Vol. 246, No.5, p 144) by Gordon Moyer is a very good review about the
adoption of the Gregorian Calendar.
- Special thanks to Matthew Bear (email@example.com) who
provided many useful comments within
a week after the page was publicly advertised. I have made some changes based
on information he has provided, but I have not had the time to make all the
- Special thanks also to
Alex Kochergin who
provided me with pretty much ALL the information here regarding the
dates and celebrations in the Eastern Orthodox Church.
- A note about different calendars: dates in the Hebrew, Islamic, Julian,
ISO, Mayan, French Revolutionary and Julian (astronomical) date calendar,
as well as their correspondence to dates in the Gregorian calendar may
be found using the "calendar" mode in the Free Software Foundation's
GNU Emacs (at least since version 19.28.1 - it may have been available in
earlier versions, too). To get to calendar mode in these versions of Emacs,
use the sequence "Meta-x calendar". This will keep you occupied if you
- Calendrical Calculations, a very useful book by Dershowitz & Reingold.
- An almanac that has a list of Easter dates through 2100 is the
World Almanac (Mahwah, NJ: Funk and Wagnalls, 1994). I suspect other
almanacs also have tables like this - I have not read all of them.
- This form was made using routines from the
by Steven E. Brenner.
Folks providing links to this page (incomplete)
This list is extremely incomplete and seriously out of date.
have links to my pages now, and it would take up an inordinate amount of
space to list all of them.
I keep this section since it lists the first folks
that I know about that provided links to this hierarchy of pages.
Thanks to everyone who provides links to this page.
- William Morris' New
Easter Dates, with dates calculated using the propsed Aleppo formula.
- Dave Goode's Orthodox Ecclesiastical
- The Royal Greenwich
Date of Easter,
- How Easter Date is
determined, an excellent, clear and colorful presentation on
determining the date of Easter, along with Mallen's algorithm.
Reints' Collection of Easter Algorithms
Simplified Easter Dating Method by R.W. Mallen, Adelaide, Australia
- A Calendar of
Jewish Feasts by B'nai B'rith
- Andrew McNab's
Perpetual Calendar Form
- Today's Calendar
and Clock Page has links to information and calendars from many different
types, including (but not limited to) Jewish, Islamic, and Chinese.
- Kalendar for Today
gives information about readings and what feast or celebration might be today.
- The Catholic Calendar
Page has beautiful and informative calendars for the current year.
Calendar FAQ, by Claus
Tondering is an excellent resource.
- The World Wide Holiday
and Festival Page by Brian Prescott-Decie contains links to pages
with both religioious and secular holidays.
Rosh Hashanah, and Passover by William H. Jefferys
- The Date of
Easter from the Anglican Diocese of Ely
of Ash Wednesday and Easter Sunday and The Date of
Easter from the Astronomical
Application Dept. of the U.S. Naval Observatory
- The Liturgy of the Hours
- General Roman
Calendar of Feasts, Solemnities, and Memorials
- Kenneth Bath's ROMCAL
Version 3, C source code for a program to generate a Roman
Catholic Calendar, including support for generating color PostScript
and HTML calendars.
- If you liked this page, please visit my American Secular Holidays Page.
The views and writings presented here are my own,
and are NOT the responsibility of Smart Net.
I have consulted many sources, and I believe that the work I have
presented is correct. However, since sometimes hardware and/or software
misbehave in subtle ways, and since I may, on occassion, mistype, or
even accidentally use wrong or mistaken sources, the following
disclaimer applies to all the pages in this hierarchy:
YOU ARE USING THESE DATES TO PLAN TRAVEL, MEETINGS, OR FOR ANY USE
REQUIRING THE EXPENDITURE OF MONEY, TIME, OR OTHER RESOURCES, PLEASE
CONSULT OTHER SOURCES TO VERIFY THE DATES OF THE VARIOUS HOLIDAYS.
Neither Marcos Montes nor anyone who owns the hardware or manages the
host machines of this home page, nor anyone who has contributed any
information that I have used on these pages, may be held financially
responsible, or responsible in any way, if these dates are wrong. The
user assumes full responsibilty for the consequences of using this
Resources and Acknowledgements |
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Related Links |
Last updated 2001 July 31.
Copyright © 1996-2001 by Marcos J. Montes.
Marcos J. Montes
My American Calendar Page.