Sun, moon or both: why religious holidays move

A solar calendar follows the seasons: the earth goes round the sun in about 365¼ days. The Gregorian calendar, used for civil life almost everywhere, is solar, and so are fixed-date feasts like Christmas.

A lunar calendar follows the moon: from one new moon to the next is about 29½ days, and twelve such months make about 354 days. The Islamic calendar is purely lunar, so its months move through the seasons, completing the circle in about 33 years.

A lunisolar calendar uses lunar months but adds an extra, leap month every two or three years, so that festivals stay in the same season. The Jewish, Hindu, Buddhist and Chinese calendars all work this way.

Jesus died at the time of Passover, a festival fixed by the Jewish lunisolar calendar. Early Christians kept Easter in relation to it. In 325 the Council of Nicaea called for all churches to keep Easter on the same Sunday. The rule later adopted everywhere is that Easter falls on the first Sunday after the first full moon on or after the spring equinox, fixed by the church at 21 March.

Western churches calculate this with the Gregorian calendar, introduced in 1582. Most Orthodox churches still use the older Julian calendar for the calculation, so their Easter, Pascha, is often one or more weeks later.

The Islamic calendar begins with the hijra, Muhammad’s migration from Mecca to Medina in 622. Each month begins with the first sighting of the thin crescent moon after sunset. Ramadan, Eid al-Fitr and Eid al-Adha therefore move about eleven days earlier every solar year. Over a lifetime, a Muslim fasts Ramadan in every season.

Because it depends on sighting, the start of a month can differ by a day between countries, and some communities follow astronomical calculation instead.

Hindu festivals follow the panchanga, a lunisolar calendar of lunar days, tithi, and lunar months, with a leap month added from time to time to keep step with the sun. Diwali falls on the new moon of Kartika (Ashvin in some reckonings), so it moves between mid-October and mid-November. Buddhist countries celebrate Vesak at the full moon of the month of Vesakha, usually in May. The Chinese calendar usually places the Lunar New Year at the second new moon after the winter solstice, between 21 January and 20 February.

The Bahá’í calendar is different again: nineteen months of nineteen days, with the new year at the spring equinox.