Muslims around the world use the Islamic calendar (also known as the Lunar or Hijri calendar) to determine the dates of religious events and observances.

The calendar is based on 12 lunar months – a new month begins when a new moon is sighted.

The Hijri calendar consists of the following 12 months in numerical order:

1. Muharram
2. Safar
3. Rabi Al-Awwal
4. Rabi Al-Thani
5. Jamada Al-Awwal
6. Jamada Al-Thani
7. Rajab
8. Shaban
9. Ramadan
10. Shawwal
11. Dhul Qadah
12. Dhul Hijjah (month of Hajj)

The calendar consists of 354 or 355 days.

The Umm al-Qura calendar is an official Islamic calendar used in Saudi Arabia, that is based on astronomical calculations. The dates are calculated in advance for nine of the twelve months in the Umm al-Qura calendar, except for Ramadan, Shawwal and Dhul Hijjah, where sighting of the moon is observed.

The Umm al-Qura calendar is based on astronomical calculations. The dates are calculated in advance for nine of the twelve months in the Umm al-Qura calendar, except for Ramadan, Shawwal and Dhul Hijjah, where sighting of the moon is observed. Whereas with the Hijri calendar, sighting of the moon is observed for every one of the 12 months.