Kindly find below two papers. The one to the right is a paper from an old Saudi calendar (which was printed before changing the criteria), showing that 1 Ramadan coincides with 08 December. Whereas the one to the left is a paper from Umm Al-Qurrah Calendar 1420 AH, showing that 1 Ramadan is 09 December 1999.

Old Saudi Calendars

The New Criteria (1423 AH - Present)

The The official website of Umm Al-Qurah Calendar mentions that if on the 29th day of the lunar month these two conditions are satisfied, then the next day is the first day of the new lunar month:-

  1. The New Moon Conjunction occurs before Sunset
  2. The Moon sets after the Sunset

The main extract from the above website is given below:

"The calculations of the Um Al-Qura calendar are based on the terms set by the distinguished Council of ministers, which adopt the location of the Holy Kaabah as the reference for all calculations, and require conjunction to occur prior to sunset (moonset occur after sunset) as conditions for the birth of the new lunar month.

The Um Al-Qura calendar is both the formal and civil calendar of the Kingdom, but it may not match the actual visibility of the crescent [moon] which is necessary to start the religious months."

Note also that the moon sighting astronomy experts are of the opinion that the Saudi Umm Al-Qurah calendar criteria does NOT guarantee the visibility of the moon. Indeed, none of the official Saudi Hilal Committee members have ever been able to sighting the moon on the 29th day of the Umm Al-Qurah calendar date in Saudi Arabia. However, quite often unreliable witness claims are accepted to match the Umm Al-Qurah date!

Related: Solar Eclipse and the Ummul Qura Calendar (pdf) | Ref: ICOP website/members (