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This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

A religious event is a dated or dateable occurrence a tradition marks as significant, whether it happens once, such as a founding revelation, a martyrdom or a council that settled a dispute, or recurs on a calendar, such as an annual festival or a seasonal observance. The type covers both kinds because the atlas is recording the occurrence itself, the thing that happened or that is marked as happening again each year, rather than the ongoing rule or custom that grew up around it.

That last distinction matters for finding things here. An Observance Rule entry records the standing practice a tradition follows, the rule that a certain day is kept a certain way; a religious event entry records the day itself, what is commemorated on it and, where it can be established, when and how the underlying occurrence actually took place. Each entry states what a tradition itself holds about the event, and separately, where scholarship differs from that account, what the historical record can and cannot confirm.

A religious event entry also records who is understood to have taken part, where it is known to have happened or where a tradition places it, and how the commemoration of it, if any exists, has changed over the centuries since, since the way an anniversary is kept today is often quite different from how it was first observed.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Recurring religious events depend on a calendar, and traditions have not agreed on how to build one. Islam runs on a purely lunar calendar of twelve lunar months with no correction for the solar year, which is why its festivals drift steadily earlier through the seasons across a person's lifetime. Judaism and traditional Chinese religion instead use lunisolar calendars, which add an extra month in some years specifically to keep their festivals anchored to the same season every time. Most Christian churches instead follow the solar Gregorian calendar, with the notable exception of several Orthodox churches that still calculate their fixed feasts on the older Julian calendar, currently thirteen days behind it.

Fixing the date of a one time historical event is a separate and often harder problem, since the earliest surviving account of a founding or a council is frequently written down decades after the fact, from memory or from an even earlier source that no longer survives. Historians of religion treat the traditional date a community keeps and the date the surviving evidence actually supports as two different questions, worth stating separately rather than silently merging into one.