Islamic (Hijri) Calendar

The Islamic, or Hijri, calendar is purely lunar: its twelve months follow the phases of the moon, each beginning with the sighting of the new crescent, and its year of about 354 or 355 days takes no account of the solar seasons. Because it is some eleven days shorter than the solar year, its months and festivals move steadily earlier through the Gregorian year, completing a full circuit in about thirty-three years, so Ramadan and the Hajj fall in every season over time. Its era, AH, counts from the Hijra, the migration of the Prophet Muhammad from Mecca to Medina in 622 CE. Four of its months are sacred, and its rhythm governs the fasts, feasts, and pilgrimage of Islam. These are the features of the Islamic calendar tradition, kept by observation of the moon and, in some usages, by calculation.

The twelve Hijri months, Muharram, Safar, Rabi al-Awwal, Rabi al-Thani, Jumada al-Ula, Jumada al-Akhirah, Rajab, Shaban, Ramadan, Shawwal, Dhu al-Qadah and Dhu al-Hijjah, alternate nominally between 30 and 29 days, though actual month lengths depend on how each is fixed. Four are traditionally sacred, Muharram, Rajab, Dhu al-Qadah and Dhu al-Hijjah, months in which warfare was restricted in pre-Islamic and early Islamic custom. In practice, Muslim-majority states differ in method: some retain local moon-sighting committees, others use the tabular Islamic calendar, a fixed 30-year arithmetical cycle with eleven leap years adding a day to Dhu al-Hijjah, and Saudi Arabia's civil Umm al-Qura calendar is calculated astronomically for administrative purposes while religious observances there still typically follow sighting announcements.

Every calendar in the world solves the same arithmetic problem, and there are only three solutions. A lunation is about 29.53 days and a tropical year about 365.2422 days, and the second is not a whole multiple of the first. A calendar can therefore follow the Moon and let its year drift against the sun, follow the sun and let its months lose contact with the Moon, or follow both and insert an extra month at intervals to keep them together.

The Hijri calendar takes the first path and takes it without compromise. Twelve lunar months, no intercalation of any kind, a year of 354 or 355 days, and consequently a steady drift of about eleven days a year against the seasons, so that Ramadan moves through the whole solar year in roughly thirty-three years. The Quran is explicit that the months are twelve and that intercalation is not to be practised, and the prohibition is understood to have ended a pre-Islamic Arabian practice of adding a month.

That makes it the only major calendar still in wide use that is purely lunar, and the drift is a feature rather than a defect: the fast and the pilgrimage pass through every season, so no part of the year is permanently easier or harder for any latitude.

A second distinguishing feature is that the month traditionally begins on the sighting of the new crescent, which is an observation and can differ between places and between authorities. Calculated versions exist and are used for civil purposes and for printed calendars, so a Hijri date may be given differently by two sources on the same day, and the difference is a matter of method rather than of error.