Hijri Calendar Guide: Islamic Months, Umm Al-Qura vs Tabular, and Key Dates

The Hijri calendar is 354 days long, has 12 lunar months, and shifts roughly 11 days earlier each Gregorian year. Understanding how it works — and how to convert dates accurately — is essential for anyone living in, working with, or planning around GCC countries and the Islamic world.

How the Hijri calendar works

The Islamic Hijri calendar is a pure lunar calendar — each month begins with the sighting of the crescent moon. Unlike the Gregorian solar calendar (365.25 days) or the Hebrew lunisolar calendar (which adds leap months to stay aligned with seasons), the Hijri calendar makes no corrections to align with the solar year. As a result, it drifts approximately 11 days earlier each Gregorian year.

This drift means Ramadan, Eid, and all Islamic observances cycle through every season over a ~33-year cycle. Ramadan in 2012 fell in August; in 2030 it will fall in January.

The calendar begins from the Hijra — the Prophet Muhammad's migration from Mecca to Medina in 622 CE. The current Hijri year is approximately 1446 AH (anno Hegirae).

The 12 Islamic months

• Muharram — the first month; Islamic New Year falls on 1 Muharram. The 10th (Ashura) is observed by both Sunni and Shia Muslims.
• Safar — the second month.
• Rabi al-Awwal — the Prophet's birthday (Mawlid) falls on 12 Rabi al-Awwal.
• Rabi al-Thani — the fourth month.
• Jumada al-Awwal — the fifth month.
• Jumada al-Thani — the sixth month.
• Rajab — a sacred month; Isra' and Mi'raj falls on 27 Rajab.
• Sha'ban — the month before Ramadan; mid-Sha'ban (Laylat al-Bara'at) is observed by many.
• Ramadan — the month of fasting; Laylat al-Qadr falls in the last 10 nights.
• Shawwal — Eid Al Fitr falls on 1 Shawwal.
• Dhul-Qa'dah — the 11th month, one of the four sacred months.
• Dhul-Hijjah — the month of Hajj; Eid Al Adha falls on 10 Dhul-Hijjah.

Umm Al-Qura vs tabular calculation

There are two main methods for computing Hijri dates mathematically:

Umm Al-Qura is the official calendar of Saudi Arabia. It is based on astronomical calculations of the moon's visibility from Mecca and is used for civil purposes in Saudi Arabia — government schedules, official holidays, and legal documents. The AlAdhan API provides accurate Umm Al-Qura data.

Tabular (arithmetic) method uses a fixed mathematical algorithm based on a 30-year cycle where 11 of the 30 years are leap years (355 days). It is deterministic — the same date always returns the same result — and is used in software, historical research, and astronomical calculations. The result may differ from the Umm Al-Qura calendar by exactly 1 day.

Neither method reflects the actual moon sighting used in some countries to declare the start of Ramadan or Eid. Local religious authorities may announce dates that differ from both calculations by a day.