The Umm al-Qura calendar is the official civil calendar of Saudi Arabia, used for administrative and business purposes. It’s based on astronomical calculations for the coordinates of Mecca (21°25′N 39°49′E) rather than actual moon sightings.

The calendar uses two main criteria:

1. The conjunction (astronomical new moon) must occur before sunset in Mecca.
2. On the evening of the 29th day, the moon must set after the sun in Mecca.

If both conditions are met, the next day is declared the first day of the new month. Otherwise, the current month continues for one more day.

While the Umm al-Qura calendar is used for civil scheduling in Saudi Arabia, religious events like the beginning of Ramadan, Eid al-Fitr, and Eid al-Adha are still determined by the moon sighting committees under the Saudi Supreme Court. The calendar provides expected dates, but final confirmation comes from actual moon sightings.

Different countries and communities use different methods to determine the Islamic calendar:

• Umm al-Qura: Astronomical calculations for Mecca
• Traditional sighting: Actual moon observation
• Tabular Islamic calendar: Fixed alternating 29/30-day months

These methods can sometimes give different results, especially at the beginning and end of Ramadan.

The Umm al-Qura calendar is highly accurate for predicting when the crescent moon should be visible from Mecca. However, actual visibility can be affected by:

• Atmospheric conditions (clouds, dust, pollution)
• Observer location and altitude
• Weather conditions

The calendar is typically accurate to within ±1 day of actual moon visibility.

The White Days refer to the 13th, 14th, and 15th of each Islamic month when the moon is full or nearly full. Fasting on these three days each month is a recommended Sunnah practice (not obligatory). The name “White Days” comes from the brightness of the moon during these nights.

The Umm al-Qura calendar is primarily used in Saudi Arabia for civil purposes. Other Gulf Cooperation Council (GCC) countries often reference it, but may use different methods for religious determinations. Many Muslim communities worldwide also use it as a reference for planning purposes, though local moon sighting often takes precedence for religious events.

The Umm al-Qura calendar was first introduced in Saudi Arabia in 1925 (1343 AH) during the reign of King Abdulaziz. The current calculation method was established in 2002 (1423 AH) by the King Abdulaziz City for Science and Technology (KACST), which continues to publish the official calendar annually.