Ordo

An open source generator for the Catholic liturgical calendar.

Status: M1 done. Liturgical seasons, movable days and lectionary cycles are available and checked against reference calendars for 2024 to 2029. Celebrations (saints, solemnities, precedence) are not implemented yet.

Principles

• Every movable day is derived from the date of Easter.
• @ordo/core contains pure functions, has no runtime dependencies, and never returns display strings.
• Calendar (country), language and readings are three separate concerns.

Product requirements and the milestone roadmap are in docs/PRD.md. Design decisions are recorded in docs/adr.

Development

Requires Bun (version pinned in .bun-version).

bun install
bun run check   # lint + typecheck + test

Example

import { easter, getAnchors, getLiturgicalTime } from "@ordo/core";

easter(2026); // "2026-04-05"

getAnchors(2026).pentecost; // "2026-05-24"

getLiturgicalTime("2026-05-25");
// { season: "ordinary-time", week: 8, holyWeek: false, date: "2026-05-25",
//   weekday: 1, sundayCycle: "A", weekdayCycle: "II" }

// Regional options (defaults follow the General Roman Calendar)
getAnchors(2026, { ascension: "sunday" }).ascension; // "2026-05-17"

Weeks are numbered by the most recent Sunday of the season. The Christmas season and the Triduum have no week number.

Contributing

All code, comments, commit messages and documentation are written in English. See AGENTS.md for conventions (they apply to human contributors too).

License

MIT © 2026-present Paroki Developers.