"They ask thee concerning the New Moons. Say: they are but signs to mark fixed periods of time in (the affairs of) men, and for pilgrimage"- The Holy Koran, Al-Baqara, Verse 189. "The number of months in the sight of Allah is twelve (in a year) ― so ordained by Him the day He created the heavens and the earth; of them four are sacred; that is the straight usage. So wrong not yourselves therein and fight the pagans all together as they fight you all together. But know that Allah is with those who restrain themselves"-
The Holy Koran, Al-Tawba, Verse 36.
Prophet Muhammad (PBUH) said: "We are an illiterate nation; we neither write, nor know accounts. The month is like this and this, i.e. sometimes of 29 days and sometimes of 30 days. " (Bukhari and Muslim).
Furthermore, in his farewell pilgrimage, the Prophet PBUH said: " Time has grown similar in form and state to the time when Allah created the heavens and the earth. A year is twelve months. Four of which are Sacred Months (Hurum). Three of the four months are successive. They are Dhul-Qa‘dah, Dhul-Hijjah, and Al-Muharram. The fourth Month is Rajab Mudar, which comes between Jumada and Sha‘ban. "
The Umm Al-Qura calendar is thus defined, adopting the Hijri lunar Islamic system of dating events and accounting days and months. It was initiated by the Second Caliph Omar Ibn Al Khattab when he established public accounts, and began numbering the years of the Islamic calendar from the year of the Hijra (July 15, 622), when the Prophet PBUH emigrated from Mecca to Medina. The Kingdom of Saudi Arabia has adopted the Hijri system as its official calendar on both an official and civilian level, as the Islamic rites (pilgrimage, fasting and Zakat) are determined by the Hijri calendar, in addition to other Islamic liturgies.
The first issue of the Um Al-Qura calendar was published in 1346H by the government press in Mecca where it was printed until 1399H. The government then decided to have it printed in its press office in Riyadh, where state of the art equipment can produce an elegant modern version of the calendar. To further develop the calendar, a supervisory committee was created, chaired by the president of KACST, with member experts in Islamic law and astronomy, to review any studies or remarks related to the calendar. The committee has been active since 1400H, making recommendations on issues pertinent to the calendar.
In 1420 H, the coordinates of the Kaaba were adopted as the basis of the Umm Al-Qura calendar. If moonset occurs after sunset at Mecca, then the day beginning at that sunset is considered the first day of the new month.