Method and sources
About these pages: computed, then checked
labs.llc’s Easter pages give the date of Easter for any year, Western and Orthodox, with the feasts that move with it. Nothing is looked up when you use them. This page says how the dates are made, what they were checked against, and what the pages cannot tell you.
What is here
- The Easter date calculator: any year from 1583 to 9999.
- Easter dates by year: a printed page for each of the 201 years from 1900 to 2100.
- How the date of Easter is calculated, worked line by line.
- The earliest and latest Easters, and how often each date occurs.
- The years Western and Orthodox Easter fall on the same day.
- The Easter egg in software and games: where the term comes from.
- And beside them, Holidays: every country’s public holidays, with its own Easter band that links here.
How the dates are made
Western Easter is computed by the Anonymous Gregorian algorithm, first printed in Nature in 1876 and also known as the Meeus/Jones/Butcher algorithm. Orthodox Easter is computed by the Julian algorithm Jean Meeus gives in Astronomical Algorithms (1991), which yields a date in the Julian calendar; that date is converted to a day number and then to the Gregorian calendar (Wikipedia, “Date of Easter”). All of it is whole-number arithmetic. No clock, time zone or network is involved in a date.
The arithmetic is written twice, in JavaScript for the pages and in Python for the program that prints them. The Python also holds a second, independent method: the golden number, the epact and the full-moon tables of The Calendar FAQ, with Easter taken as the Sunday after the tabulated moon.
The cross-checks
| Check | Years | Compared | Disagreements |
|---|---|---|---|
| Day numbers vs Python datetime (every day) | 1583-01-01 to 9999-12-31 | 3,074,246 | 0 |
| Julian-to-Gregorian shift formula vs day numbers | 1583–9999 | 8,417 | 0 |
| Western: algorithm vs epact-table method | 1583–9999 | 8,417 | 0 |
| Orthodox: algorithm vs golden-number-table method | 1583–9999 | 8,417 | 0 |
| Invariants (Sunday, range, first Sunday after the full moon, epact = 23 - h, shift) | 1583–9999 | 8,417 | 0 |
| python-dateutil 2.9.0.post0 (Western and Orthodox) | 1583–4099, the range it documents | 2,517 | 0 |
| easter-core.js vs Python (dates, day numbers, Julian date, shift, golden number, epact, full moon) | 1583–9999 | 8,417 | 0 |
| easter-core.js vs Python (every feast, every printed sentence part, every algorithm step) | 1900–2100 | 201 | 0 |
| easter-core.js vs Python (the words: long dates, weeks apart, day counts) | 1583–9999 | 8,417 | 0 |
| US Census Bureau: its ASCII file vs its own HTML table | 1600–2099 | 500 | 0 |
| US Census Bureau table vs computed Western Easter | 1600–2099 | 500 | 0 |
| US Census Bureau table, the printed years | 1900–2099 | 200 | 0 |
| 5ko.free.fr side-by-side tables (Western and Orthodox, Julian and Gregorian dates) | 1801–2200 | 400 | 0 |
| 5ko.free.fr tables, the printed years | 1900–2100 | 201 | 0 |
| Wikipedia 'List of dates for Easter' table (Gregorian and Julian Easter) | 2016–2036 | 21 | 0 |
| Wikipedia 'Date of Easter' worked examples, every intermediate value | — | 10 | 0 |
| Calendar FAQ printed golden numbers, epacts and dates | — | 6 | 0 |
| Wikipedia 'List of dates for Easter' statements (earliest, latest, same-day years) | — | 6 | 0 |
| Wikipedia infobox dates of Ash Wednesday and Clean Monday | — | 7 | 0 |
| Every feast falls on the weekday in its name | 1583–9999 | 8,417 | 0 |
Run on September 28, 2026, when these pages were printed. A single disagreement stops the build and nothing is published. Published tables were read on September 28, 2026: the US Census Bureau table covers Western Easter from 1600 to 2099; the 5ko.free.fr tables cover Western and Orthodox Easter in both calendars from 1801 to 2200; Wikipedia’s list of dates for Easter covers 2016 to 2036. The year 2100 is past the end of the Census table and rests on the 5ko.free.fr tables, python-dateutil and the second method.
The feasts and their offsets
| Feast | From Easter | Basis |
|---|---|---|
| Shrove Tuesday | −47 days | The day before Ash Wednesday. Wikipedia, “Shrove Tuesday” |
| Ash Wednesday | −46 days | 46 days before Easter Sunday. Wikipedia, “Ash Wednesday” |
| Palm Sunday | −7 days | The Sunday before Easter. Wikipedia, “Palm Sunday” |
| Maundy Thursday, Good Friday, Holy Saturday | −3 days | The Thursday, Friday (−2) and Saturday (−1) before Easter Sunday, counted on the calendar. |
| Easter Monday | +1 day | The day after Easter Sunday. |
| Ascension | +39 days | The fortieth day of Easter, counting Easter Sunday. Wikipedia, “Feast of the Ascension” |
| Pentecost | +49 days | The 49th day after Easter. Wikipedia, “Pentecost” |
| Clean Monday (Orthodox) | −48 days | 48 days before Pascha Sunday. Wikipedia, “Clean Monday” |
Sources read September 28, 2026. The build also confirms that every feast lands on the weekday in its name for each of the 8,417 years the calculator accepts, and compares Ash Wednesday and Clean Monday with the dates printed in the two Wikipedia articles.
What these pages cannot tell you
- What your church keeps. Observance varies by church and by country. Some churches keep Ascension on the following Sunday, and some follow other reckonings of Easter altogether. The dates here are the Gregorian and the Julian computus, nothing more.
- Whether a day is a holiday. That is each country’s law. Holidays lists each country’s public holidays, off the open Nager.Date database.
- The moon in the sky. The Paschal full moon is reckoned from tables and can differ from the astronomical full moon by a day or two (The Calendar FAQ).
- The far future. Dates centuries ahead are the present rules carried forward. The rules are conventions and could change (Wikipedia, “Date of Easter”).
- Years before 1583. The Gregorian calendar went into effect in October 1582 (Wikipedia, “Gregorian calendar”); the calculator starts with the first Easter after it.
Privacy
The calculator runs in your browser. The pages’ own code makes no request to any data service, and the year you type is not sent anywhere and is never put into the address. Your choice of paper or dark edition is remembered in your browser, in local storage and in a cookie named labs_edition, as it is everywhere on labs.llc. labs.llc’s Google Analytics counts page views and, the first time the calculator is used on a visit, that it was used; it is never told the year.
Colour
| Colours | Used for | Contrast |
|---|---|---|
#A16207 on #ffffff | Accent on paper | 4.92 : 1 |
#854D0E on #ffffff | Accent as small text on paper | 6.85 : 1 |
#A16207 on #FAFAFA | Accent on the tinted band | 4.72 : 1 |
#854D0E on #FAFAFA | Accent as small text on the tinted band | 6.56 : 1 |
#FDE047 on #0C1116 | Accent on the dark ground | 14.38 : 1 |
#FDE047 on #0F151B | Accent on the dark tinted band | 13.93 : 1 |
#FDE047 on #121920 | Accent on a dark card | 13.44 : 1 |
Measured by the build with the WCAG relative-luminance formula; under 4.5 : 1 the build stops.
Who publishes this
These pages are part of labs.llc. They began in September 2026 on an address of their own, easteregg.llc; every page there has its copy here, on the same slug. Corrections are welcome at labs@labs.llc.
Sources
- Wikipedia, “Date of Easter” (revision 1366577142). Read September 28, 2026.
- Wikipedia, “List of dates for Easter” (revision 1377204128). Read September 28, 2026.
- Claus Tøndering, “The Calendar FAQ: Easter”. Read September 28, 2026.
- US Census Bureau, “Easter Dates from 1600 to 2099”. Read September 28, 2026.
- 5ko.free.fr, “Side-by-side Easter calendar reference”. Read September 28, 2026.
- Wikipedia, “Gregorian calendar” (revision 1374418208). Read September 28, 2026.
- Wikipedia, “Ash Wednesday” (revision 1369573734). Read September 28, 2026.
- Wikipedia, “Shrove Tuesday” (revision 1370877495). Read September 28, 2026.
- Wikipedia, “Palm Sunday” (revision 1376710312). Read September 28, 2026.
- Wikipedia, “Feast of the Ascension” (revision 1371917210). Read September 28, 2026.
- Wikipedia, “Pentecost” (revision 1375874373). Read September 28, 2026.
- Wikipedia, “Clean Monday” (revision 1358857344). Read September 28, 2026.
- Wikipedia, “Easter egg (media)” (revision 1374491931). Read September 28, 2026.
- Chris Baker, “How One Man Invented the Console Adventure Game”, WIRED, March 13, 2015. Read September 28, 2026.
- Kate Willaert, “Make Love Not War: Talking With The Creator Of The First Software Easter Egg”, A Critical Hit!, May 23, 2021. Read September 28, 2026.
- Kate Willaert, “The True First Easter Egg: Ready Player One Was Wrong”, A Critical Hit!, April 4, 2021. Read September 28, 2026.