Islam's lunar calendar drifts eleven days every solar year
The Hijri calendar counts twelve moon-based months, so Ramadan and Hajj rotate through every season over a thirty-three-year cycle. Its epoch marks the Prophet's 622 CE migration from Mecca to Medina, when the first Muslim community formed.
The Hijri calendar is a lunar system of twelve months totaling 354 or 355 days, used to set Islamic holidays and rituals. Most Muslim-majority countries use the Gregorian calendar for civil affairs while keeping Hijri dates for religious life. Years are labeled AH—Anno Hegirae—from an epoch tied to Muhammad's 622 CE Hijrah from Mecca to Medina. As of June 2026, the current Islamic year is 1448 AH.
Months traditionally begin when observers sight the crescent moon, giving each month twenty-nine or thirty days. Four months are sacred, when fighting was forbidden: Rajab alone, then Dhu'l-Qa'da, Dhu'l-Hijja, and Muharram consecutively. An Islamic day runs from sunset to sunset. Friday, Yawm al-Jum'a, is the gathering day for noon prayer and serves as a weekly rest in many countries.
Because twelve lunar months fall about 10.875 days short of a solar year, Hijri dates shift roughly eleven days earlier each year relative to the equinoxes. The Astronomical Almanac notes the twelve-month cycle regresses through seasons over about thirty-three solar years. This drift is accepted in pure lunar calendars, unlike lunisolar systems that add intercalary months.
Pre-Islamic Arabia had complex calendar customs. Scholars debate whether "Nasī'" meant shifting sacred months within a year or inserting extra months to align with seasons. The Qur'an prohibited Nasī' around the tenth year after Hijra, announced during Muhammad's farewell sermon on 9 Dhu'l-Hijja AH 10. Twelver Shia count the new year from Rabi' al-Awwal rather than Muharram, so events like the battle of Karbala are dated 60 AH by Shia sources and 61 AH by Sunni ones.
Source: Islamic calendar
