Church Calendar (Calapi) API

Free Catholic liturgical calendar API with no key: today's celebrations, liturgical season, colour and feast rank for any date, in six languages. Tested example included.

No API key requiredCORS enabledFree tier

Endpoint tested and returned HTTP 200 on 2026-08-21

What is the Church Calendar (Calapi) API?

Calapi is a free, key-free API for the Roman Catholic liturgical calendar. For any date it returns the liturgical season, the week within that season, the day's celebrations with their rank and liturgical colour, and supports national calendar variants in six languages.

The liturgical calendar is a moving-feast problem: Easter's date is computed astronomically, and roughly a third of the year's observances shift with it, cascading through Lent, Eastertide and Ordinary Time. On top of that, saints' feasts vary by country and can be outranked by a season. Calapi implements the whole ruleset so you do not have to.

The `rank_num` field is the piece that makes this genuinely usable rather than merely informative. Celebrations are numerically ranked — solemnities highest, then feasts, memorials and optional memorials — which is exactly what you need when two land on the same day and you must decide which takes precedence. The `colour` field pairs with it for vestment and UI colour. National calendars are selected by path segment, so the same date can correctly return different celebrations for different countries.

Quick facts

Base URL: http://calapi.inadiutorium.cz/api/v0
Authentication: No API key or account. Open source and free for any use. Note that this endpoint is HTTP-only, which matters if you are calling it from an HTTPS page.
Rate limit: No published limit. The calendar is deterministic, so compute once and cache indefinitely.
Pricing: Free and open source.
CORS: Enabled — callable directly from browser JavaScript
Official docs: Read the docs

How to use the Church Calendar (Calapi) API

Every request below was executed against the live API on 2026-08-21, and the response shown is the real body it returned — not an illustration.

1. Fetch today's liturgical celebration

GET http://calapi.inadiutorium.cz/api/v0/en/calendars/default/today

curl

curl 'http://calapi.inadiutorium.cz/api/v0/en/calendars/default/today'

JavaScript (fetch)

const res = await fetch("http://calapi.inadiutorium.cz/api/v0/en/calendars/default/today");
if (!res.ok) throw new Error(`Request failed: ${res.status}`);
const data = await res.json();
console.log(data);

Python (requests)

import requests

res = requests.get("http://calapi.inadiutorium.cz/api/v0/en/calendars/default/today", timeout=20)
res.raise_for_status()
print(res.json())

Response — HTTP 200

{
  "date": "2026-08-21",
  "season": "ordinary",
  "season_week": 20,
  "celebrations": [
    {
      "title": "Saint Pius X, pope",
      "colour": "white",
      "rank": "memorial",
      "rank_num": 3.1
    }
  ],
  "weekday": "friday"
}

Parameters

Parameter	Type	Required	Description
`lang`	path segment	Required	Language code: `en`, `cs`, `fr`, `it`, `la` or `es`. `en`
`calendar`	path segment	Required	National calendar variant, or `default` for the general Roman calendar. `default`
`date`	path segment	Optional	`today`, or a `/{year}/{month}/{day}` path for a specific date. A year and month alone returns the whole month. `today`

Response fields

datestring: The date in ISO format.
seasonstring: Liturgical season: `ordinary`, `advent`, `christmas`, `lent` or `easter`.
season_weekinteger: Week number within the season, which is how liturgical readings are indexed.
weekdaystring: Day of the week in the requested language.
celebrationsarray: The day's observances. Usually one entry, but several on days where optional memorials coincide.
celebrations[].titlestring: Name of the celebration in the requested language.
celebrations[].colourstring: Liturgical colour — `white`, `red`, `green`, `violet` or `rose`.
celebrations[].rank / rank_numstring, float: Rank name and its numeric precedence. Higher `rank_num` wins when celebrations collide.

What you can build with the Church Calendar (Calapi) API

Show the day's liturgical celebration on a parish website
Build a Catholic calendar or daily readings application
Colour a calendar UI by liturgical season automatically
Generate a printable annual liturgical calendar for a specific country

Common errors and how to fix them

404

Unknown language, calendar name or malformed date path.

Fix: The path order is language, then calendar, then date. National calendar names are specific — check the index endpoint for the list.

Mixed content blocked

The API is served over HTTP only.

Fix: Calling it directly from an HTTPS page is blocked by the browser. Proxy through your own backend, which you should do anyway to cache the results.

Multiple celebrations returned

Not an error — optional memorials can coincide.

Fix: Sort by `rank_num` descending and take the highest, or present the choice, which is what the liturgical rules actually intend.

Church Calendar (Calapi) API — frequently asked questions

Is the Catholic calendar API free?

Yes, free with no API key or account, and the project is open source. Note that it is served over HTTP only, so proxy it rather than calling it directly from an HTTPS page.

What does rank_num mean?

It is the numeric precedence of a celebration — solemnities rank highest, then feasts, memorials and optional memorials. When two observances fall on the same day, the higher `rank_num` takes precedence, which is exactly the decision the field exists to let you make.

Does it handle different countries' calendars?

Yes. National calendar variants are selected by a path segment, so the same date can correctly return different celebrations for different countries — local saints and national feasts differ considerably.

Why do liturgical dates move each year?

Because Easter is computed astronomically and about a third of the year's observances are anchored to it, cascading through Lent, Eastertide and Ordinary Time. That computation is the main reason to use this API rather than a static table.

Tools that pair with this API

Timestamp Converter

Convert timestamps to human-readable dates and dates back to timestamps. Auto-detects seconds vs milliseconds, shows local, UTC and ISO 8601 formats.

Date Difference Calculator

Calculate the exact difference between two dates in years, months and days, plus total days, weeks, hours and minutes. Free, fast and private.

Week Number Calculator

Find the ISO 8601 week number for any date, plus the Monday-to-Sunday date range of that week and the day of the week. Free and 100% in-browser.

Church Calendar (Calapi) is an independent third-party service and is not affiliated with ByteTools or ByteVancer. Details on this page were verified on 2026-08-21; always check the official documentation before relying on this API in production, as terms and limits can change.