Set by an astronomical event or computus

This category holds the festivals and holy days of the religious calendars: fixed and movable feasts, fasts, new years, solstice and equinox observances, and commemorations of events and figures. Each entry records the tradition, the calendar by which the date is reckoned, and what the day marks.

Holy days give a religion its year, a repeating cycle of remembrance and renewal that structures communal life. Some commemorate an event held to be historical or mythic; others mark a turn of the season or the calendar; others are set aside for fasting, penitence or joy.

Because traditions use different calendars, the same festival can fall on different dates from year to year, and movable feasts (Easter, the Islamic festivals, the Hindu festivals set by lunar dates), shift against the civil calendar in ways the entries explain. The reckoning of dates is itself part of a festival's character, tying it to the calendars gathered elsewhere in the atlas.

The holidays group by kind (feasts, fasts, festivals, high holy days and observances), so a reader can compare, for instance, the major fasts of several traditions together.