# Easter Date Calculator

> Computes the date of Western (Gregorian) Easter Sunday for any year with the Meeus/Jones/Butcher algorithm and derives Ash Wednesday, Palm Sunday, Good Friday, Ascension Day and Pentecost; also gives Orthodox Easter from the Julian computus converted to the Gregorian calendar.

- Calculator id: `easter-date` · Category: Time & Geography (`time`) · Tool name: `calculate_easter_date`
- Canonical page: https://tttkmbb.com/time/easter-date · This document: https://tttkmbb.com/time/easter-date.md · JSON definition: https://tttkmbb.com/time/easter-date.json

## Purpose

Computes the date of Western (Gregorian) Easter Sunday for any year with the Meeus/Jones/Butcher algorithm and derives Ash Wednesday, Palm Sunday, Good Friday, Ascension Day and Pentecost; also gives Orthodox Easter from the Julian computus converted to the Gregorian calendar.

**Use when:** You need Easter or the movable feasts tied to it (Lent, Holy Week, Ascension, Pentecost) for a year, including the Orthodox date.

**Do not use when:** You need fixed-date holidays or non-Christian movable holidays (Passover, Ramadan, Diwali), or dates before 1583 when the Gregorian calendar did not exist.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `year` | integer |  | required | Gregorian calendar year. (min 1583, max 4099) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `easter_sunday` | date |  | Western (Gregorian) Easter. |
| `ash_wednesday` | date |  | Easter − 46 days; start of Lent. |
| `palm_sunday` | date |  | Easter − 7 days. |
| `good_friday` | date |  | Easter − 2 days. |
| `ascension_day` | date |  | Easter + 39 days (a Thursday). |
| `pentecost_sunday` | date |  | Easter + 49 days. |
| `orthodox_easter` | date |  | Eastern Orthodox Easter (Julian computus) expressed as a Gregorian date. |
| `orthodox_offset_days` | integer | days | Days by which Orthodox Easter follows Western Easter (0 when they coincide). |
| `golden_number` | integer |  | Position of the year in the 19-year Metonic cycle: (year mod 19) + 1. |

## Formula

`Meeus/Jones/Butcher: a = Y mod 19, b = ⌊Y/100⌋, c = Y mod 100, d = ⌊b/4⌋, e = b mod 4, f = ⌊(b+8)/25⌋, g = ⌊(b−f+1)/3⌋, h = (19a+b−d−g+15) mod 30, i = ⌊c/4⌋, k = c mod 4, l = (32+2e+2i−h−k) mod 7, m = ⌊(a+11h+22l)/451⌋; month = ⌊(h+l−7m+114)/31⌋, day = ((h+l−7m+114) mod 31)+1. Orthodox: Meeus Julian algorithm, then Julian date + (⌊Y/100⌋ − ⌊Y/400⌋ − 2) days`

Easter is the first Sunday after the ecclesiastical full moon on or after 21 March; the tabular algorithms reproduce the Gregorian and Julian computus exactly for all years. The Julian-to-Gregorian shift is 13 days for 1900–2099.

## Data Sources

- Date of Easter – Anonymous Gregorian algorithm and Meeus's Julian algorithm (Wikipedia) — https://en.wikipedia.org/wiki/Date_of_Easter (reference, retrieved 2026-09-24)
- USNO Astronomical Applications – The Date of Easter — https://aa.usno.navy.mil/faq/easter (government, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/easter-date?year=…`
- `POST https://tttkmbb.com/api/v1/calculate/easter-date` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/easter-date · OpenAPI operationId `calculate_easter_date` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "easter-date", "inputs": {…}}`

## Example

- 2026: inputs `{"year":2026}` → `{"easter_sunday":"2026-04-05","ash_wednesday":"2026-02-18","palm_sunday":"2026-03-29","good_friday":"2026-04-03","ascension_day":"2026-05-14","pentecost_sunday":"2026-05-24","orthodox_easter":"2026-04-12","orthodox_offset_days":7,"golden_number":13}`
- 2025 (Western and Orthodox Easter coincide): inputs `{"year":2025}` → `{"easter_sunday":"2025-04-20","orthodox_easter":"2025-04-20","orthodox_offset_days":0,"ash_wednesday":"2025-03-05"}`

```
GET https://tttkmbb.com/api/v1/calculate/easter-date?year=2026
```

## Limitations

You need fixed-date holidays or non-Christian movable holidays (Passover, Ramadan, Diwali), or dates before 1583 when the Gregorian calendar did not exist. Easter is the first Sunday after the ecclesiastical full moon on or after 21 March; the tabular algorithms reproduce the Gregorian and Julian computus exactly for all years. The Julian-to-Gregorian shift is 13 days for 1900–2099. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Why do Western and Orthodox Easter differ?**

Orthodox churches keep the Julian calendar and its older lunar tables, so their Easter falls 0, 1, 4 or 5 weeks after the Gregorian date; the two coincide in years such as 2017 and 2025.

**What is the possible range of dates?**

Western Easter falls between 22 March and 25 April; Orthodox Easter between 4 April and 8 May (Gregorian) during the 20th–21st centuries.

## Related

- [Days Until Calculator](https://tttkmbb.com/everyday/days-until.md) — Countdown from today to the Easter date.
- [Day of the Week Calculator](https://tttkmbb.com/everyday/day-of-week.md) — Weekday and ISO week of any of the feast dates.
- [Add or Subtract Days Calculator](https://tttkmbb.com/everyday/add-days.md) — Shift a feast date by a number of days.