Marking Time: How Religious Calendars Set Their Dates

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.