The short answer
The default is the Revised Julian reckoning — what most people call the New Calendar — with the Byzantine lectionary and the fasting rule as it is generally kept in the Greek and Antiochian tradition and in the Orthodox Church in America. The toggle switches the fixed feasts to the Julian reckoning. Pascha and everything that moves with it are the Orthodox date on both.
The two calendars, and what the toggle actually changes
The difference is only in the fixed feasts — the ones tied to a date rather than to Pascha. Between 1900 and 2099 the Julian calendar runs thirteen days behind the civil one, so a parish on the old reckoning keeps the Nativity on what the civil calendar calls January 7. The feast has not moved; the civil date has.
What the toggle does not change is Pascha. Orthodox Pascha is computed the same way in both cases — the Julian computus, converted to a civil date — which is why it usually falls later than the Western Easter. Everything that hangs off Pascha moves with it: the Triodion, Great Lent, Pentecost, the Apostles' Fast.
Where the commemorations come from
The skeleton of the calendar — which saints are kept on which Menaion date, and what is appointed to be read — comes from the fixtures of orthocal, which are published under the MIT licence. That gives 1,716 commemorations across the 366 days.
The prose in those fixtures is not used. The saints' lives in orthocal come from Abbamoses.com and are used there under a personal permission that does not extend to this site. Every line of description on the day pages here was written for this site instead. Where a commemoration has a life written for it, the day page links it; where it has not — which is true of most of them — the page shows only what the title itself attests, and says no more. That is why so many entries are short. It is a limit of the record, not an omission.
Where the appointed readings come from
The readings are the Byzantine lectionary as carried in those same fixtures: the Gospel and Epistle of the day from the movable cycle, together with the Old Testament readings appointed at Vespers on feasts and through Lent.
Readings link into the Scripture pages on this site wherever the reference can be resolved to a single passage. A reading that names several separate passages at once — the ones the books call composite — is shown as plain text rather than linked somewhere misleading.
Fasting
The fasting notes are the general rule, not a ruling. Orthodox practice varies by jurisdiction, by monastery, by parish and by what your own priest has told you, and the discipline is adjusted for the sick, the pregnant, the travelling and the young. What is shown here is the shape of the fast; what you should actually do is a question for your priest and not for a website.
Known limits
- The calendar covers the fixed Menaion cycle and the movable cycle around Pascha. It does not carry the full Octoechos tone cycle, or the daily variable hymns.
- Local and jurisdictional commemorations vary. A saint kept in your parish may not appear here, and the reverse.
- Where two traditions keep a saint on different days, the date follows the fixtures rather than adjudicating between them.
If it disagrees with your parish
Your parish is right. This is a reference, assembled carefully and stated openly, but it is not anyone's liturgical authority, and it is not published by a parish, a diocese or a jurisdiction. If something here is actually wrong rather than merely different, that is worth knowing — write and it gets fixed.