CAIRO — Nearly a billion and a half Muslims worldwide this week begin the annual holy month of Ramadan, a time of prayer and fasting that commemorates the divine revelation received by the Prophet Mohammed (PBUH).

Ramadan is the ninth month of the Muslim Hijri calendar, during which the faithful abstain from eating, drinking, smoking and having sex during daylight and, in the evening, eat small meals and visit friends and family.

It is a month of piety, alms-giving and fasting in order to instil the body and spirit with the rigours of abstinence, a time of worship and contemplation and to strengthen family and community ties.

Observing Ramadan is one of the five pillars of Islam, the others being the shahada or profession of faith, the obligation to pray five times a day, the giving of alms or zakat and going on pilgrimage to Mecca, the hajj.

The onset of Ramadan, which is determined by observing the crescent moon or by astronomical calculations, can vary from country to country.

Under the Hijri calendar, which has 11 days fewer than the Gregorian, Ramadan will begin this year -- the year 1431 -- on August 10 or 11.

The first day of the holy month is decided by the sighting of the crescent moon by the naked eye. Theologians and scholars gather every year to determine the onset of Ramadan, which varies across the globe according to location.

All post-puberty Muslim believers are expected to fast during Ramadan, and even though children are exempt, it is recommended that they become accustomed to the practice progressively.

Women who are pregnant or breast-feeding, the sick and travellers all have the right not to observe the fast, but they must do so as soon as they are able.

It is traditional to break the fast after sunset with a meal known as iftar, consisting of dates and goat's milk, as the Prophet is said to have done. The last meal before dawn is known as suhur.

The holy month ends with feasting and gifts on Islam's biggest festival, Eid al-Fitr.

Pilgrims flock to Islam's holiest sites at Mecca and Medina in Saudi Arabia during Ramadan, especially on the last 10 days of the month. The holiest night is on day 27, marking the revelation of the Koran to the Prophet in 610 AD.