Dominical letter calculator
2024 has the dominical letters G and F: G marks the Sundays of January and February, F those from March onward, because 1 January fell on a Monday. Enter any year from 1 to 9999 to get its letter or pair. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Year
- 2,024
- Dominical letter
- GF
- Letters
- G, F
- January letter
- G
- March letter
- F
The dominical letter is a classical calendar label from A through G that marks which letter in the repeating weekly cycle falls on Sundays for a given year. Days of the year are written with letters A, B, C, D, E, F, G, A, B again, starting with A on the first of January. The letter that lands on Sunday is the dominical letter of that year. Leap years need two letters because the extra day of February 29 shifts every later weekday by one step: the first letter covers January and February, and the second letter covers March through December. Historians, liturgy planners, perpetual-calendar makers, and genealogy tools still quote these letters when reconstructing which weekday pattern a year used. Non-integer years are rejected with a clear validation error.
How the dominical letter is defined and computed
Label every calendar day with a repeating seven-letter cycle A through G, assigning A to the first of January, B to the second, and so on. Sundays share one common letter for a common year; that shared letter is the dominical letter. If the first of January is a Sunday, Sundays fall on A days and the letter is A.
If the first of January is a Monday, the first Sunday of the year falls on a G day, so the letter is G. The remaining weekdays map cleanly to F, E, D, C, and B.
In a leap year the insertion of February 29 advances every subsequent date by one weekday relative to the letter stream used before the leap day, so tradition records two letters: the January–February letter from the first of January, and the March–December letter which is one step earlier in the A–G cycle (A steps back to G). This capability implements that definition with proleptic Gregorian leap rules—divisible by four, except century years unless divisible by four hundred—and Sakamoto’s closed-form weekday for the first of January.
Century non-leap years such as 1900 correctly keep a single letter, while century leap years such as 2000 correctly produce the two-letter form.
Where dominical letters show up in real work
Liturgical and ecclesiastical calendars historically index movable feasts and Sunday cycles through the dominical letter, so parish software and research notes still need a trustworthy oracle for modern and historical Gregorian years. Genealogy projects reconstructing baptism or marriage registers often know only a year and a weekday mention; recovering the dominical letter narrows which perpetual-calendar template applies. Museum and archive education programs build interactive perpetual calendars and need stable letter tables without depending on each visitor’s local time zone.
If you later need only a leap-year list between two bounds, day-of-year conversion, or Orthodox Easter dates, treat those as separate search intents and landings rather than overloading this dominical-letter page.
What you can do with it
Liturgical and perpetual calendars
Look up the A–G Sunday letter (or leap pair) for any Gregorian year when building feast tables, parish planners, or printable perpetual calendars.
Genealogy and archive reconstruction
Recover which weekday pattern a historical year used so partial register clues can be matched to a consistent calendar template.
FAQ
What is a dominical letter?
It is the letter from A to G that falls on Sundays when days of the year are labeled A–G starting with A on 1 January. Leap years use two letters: one for January–February and one for March–December.
Why do leap years have two letters?
February 29 inserts an extra day, shifting every later weekday by one step relative to the letter cycle. The first letter covers January–February; the second (one step earlier in A–G) covers March–December.
Which calendar rules does this use?
Proleptic Gregorian: a year is a leap year if divisible by 4, except century years that are not divisible by 400.
What happens if the year is not an integer?
Years must be whole numbers from 1 to 9999; floats, empty values, and out-of-range years are rejected and not charged.
What does it cost?
The browser tool is free and runs locally.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/date/dominical-letter \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"year":2024}'const res = await fetch("https://api.kit.forhosting.com/date/dominical-letter", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"year": 2024
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/date/dominical-letter",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"year": 2024
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/date/dominical-letter", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"year":2024}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"year":2024}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/date/dominical-letter", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"year": 2024
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "date.dominical_letter",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
This calculator applies proleptic Gregorian leap rules and a pure weekday formula for the first of January, so identical years always produce the same JSON without depending on the host clock or time zone.
What you send and what you get back
Send one required integer field named year in the inclusive range from one through nine thousand nine hundred ninety-nine. Classic checks include two thousand twenty-four yielding G then F, two thousand as B then A, two thousand one as G alone, and two thousand twenty-three as A alone. Missing values, empty strings, booleans, floats such as twenty twenty-four point five, NaN, Infinity, and years outside that range fail with invalid_input and are not charged. On success the response echoes the cleaned year, a boolean leap flag under Gregorian rules, dominical_letter as a one-character string for common years or a two-character string for leap years, and a letters array with the same content split into one or two elements. For convenience the payload also includes january_letter and march_letter—identical outside leap years—plus the English weekday name and zero-based index of the first of January, a calendar tag set to Gregorian, a short human rule string, and a compact formula string documenting the map from weekday index to AGFEDCB. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.
Where dominical letters show up in real work
Because this capability is a pure parse engine, API pricing is the published base of $0.002 per successful item and failed validation does not consume balance.
Integration use cases
Date-library fixtures and teaching demos
Generate stable dominical-letter tables to cross-check leap handling and weekday math in libraries, contests, and classroom answer keys.