A calendar that follows only the moon
The Hijri calendar is the lunar calendar of Islam. It has twelve months, each beginning with the sighting, or calculation, of the new crescent moon, and it never adds an extra month to keep in step with the seasons. This sets it apart from lunisolar calendars such as the Jewish and traditional Chinese ones, and from the solar Gregorian system.
Because each month is 29 or 30 days, a year of twelve months lasts 354 or 355 days. That is about eleven days shorter than a solar year. As a result, any date in the Hijri calendar falls about eleven days earlier in the solar year each time round, and the months move through all the seasons. A full circuit takes roughly 32 to 33 solar years.
The twelve months
The months, in order, are Muharram, Safar, Rabi al-awwal, Rabi al-thani, Jumada al-ula, Jumada al-thani (or al-akhira), Rajab, Shaban, Ramadan, Shawwal, Dhu al-Qada and Dhu al-Hijja. Their spellings vary in English because of different transliteration systems.
Ramadan, the ninth month, is the month of fasting. Dhu al-Hijja, the twelfth, includes the pilgrimage to Mecca. These two are among the practices summarised under the Five Pillars, and because the calendar moves, both fall at a different season from one decade to the next. A person might fast in short winter days in one period of life and in long summer days in another.
Four months are known as sacred months, in which fighting is traditionally prohibited: Muharram, Rajab, Dhu al-Qada and Dhu al-Hijja. Three of them are consecutive and one stands alone.
The Quranic basis for twelve
The standard reference for the number twelve is in the ninth surah of the Quran, At-Tawba, verse 36. It states that the number of months with God is twelve, and it is followed by the sacred-months idea. The next verse, 37, is generally understood as condemning nasi’, a practice of postponing or shifting months. Classical commentators and modern scholars connect this to a pre-Islamic Arabian custom of adding or moving months so that the sacred times fit the seasons, though the exact workings of the older practice are not fully known.
The result, in the traditional view, is that a thirteenth month is not allowed, and the calendar stays purely lunar. The number twelve here is a rule drawn from scripture, not a numerical symbol of the kind found in other traditions.
Starting point: the Hijra
The Hijri year count starts from the Hijra, the migration of the Prophet Muhammad and his followers from Mecca to Medina in 622 CE. According to the traditional account, the numbering of years was introduced during the caliphate of Umar, some years after Muhammad’s death. The first day of the first year, 1 Muharram, falls in mid-July 622 CE in the Julian calendar, which is a convention chosen for the era rather than the date of the migration itself, as the migration took place a couple of months later.
Years are written with “AH”, for anno Hegirae, in European usage. In late 2026 the Hijri year is 1448, a number lower than a simple count of Gregorian years would suggest, since Hijri years are shorter.
Days begin at sunset
In the Islamic reckoning, the day begins at sunset rather than at midnight, so a date in the Hijri calendar starts the evening before the corresponding Gregorian day. The weekly cycle is unaffected, and Friday is the day of the main congregational prayer. The origin of the seven-day week itself is covered separately; the Hijri calendar simply uses the standard week.
Seeing the moon or calculating it
A month begins when the crescent is sighted, or when it is determined by calculation. Communities differ in how they decide. Some rely on local sighting, some on sighting anywhere in a wider region, and some on astronomical calculation. Saudi Arabia has used a calculation-based civil system known as Umm al-Qura, and tabular schemes exist that use fixed patterns of 29 and 30 day months over a 30-year cycle. These produce results that can differ by a day from observed dates. As a consequence, the start of Ramadan or the dates of the two main festivals can differ between countries by a day.
Numbers in the Arabic writing of dates
Because Arabic letters also had numerical values, historians sometimes meet Hijri dates hidden in verses or inscriptions, written as chronograms in which the letters add up to the year. The system of abjad numerals is explained under that name. This shows how a calendar and a numeral system can interact, though it is a literary custom and not part of the calendar rules.
Why it matters for the study of calendars
The Hijri calendar is the clearest living example of a calendar that accepts the drift between lunar and solar time and builds ritual around the movement. The twelve lunar months, the 354-day year and the shifting seasons are tied closely together: religious life follows the moon, and the solar year is handled by other calendars when needed, for agriculture and taxation, as many Muslim-majority societies have done at different times.