The Umm al-Qura calendar is a calculated lunar calendar that was created by Saudi Arabia. It is also followed by several neighboring states on the Arabian Peninsula such as Bahrain and Qatar, as well as organizations in North America and Europe.
The Umm al-Qura calendar adopts the Hijri lunar Islamic system of dating events and accounting days and months.
The dates for the calendar are determined at the Institute of Astronomical & Geophysical Research of the King Abdulaziz City for Science and Technology (KACST) in Riyadh.
Under the current rules the lunar month will always start after the astronomical new moon and with the lunar crescent above the horizon at Mecca (often just barely so). Still, in most cases (about 75%) the lunar crescent will not be visible to the naked eye on the first evening of the lunar month as seen from Mecca (or elsewhere in Saudi Arabia).