MECCA, Saudi Arabia (AP) _ The world's 1 billion Muslims feasted at celebrations Sunday on the eve of the holy fasting month of Ramadan.

Ramadan, one of the most important observances of Islam, marks the ninth month of the Muslim calendar, when the Prophet Mohammed began to receive Islam's holy book, the Koran, from God. The period of fasting begins at dawn Monday in most of the Islamic world.

The faithful refrain from food, drink and temporal pleasures between dawn and dusk. Generous donations for the needy are a tradition. Worshippers flock to mosques for prayers or Koranic recitations and for sermons.

Fasting is obligatory for all mature and fit Muslims, but can be broken for traveling, sickness and pregnancy.

The pattern of business life adapts to the hours of the fast. Governments decree shorter working hours, restaurants and cafeterias close by day, and shops open at night and close during the day. Many Muslims spend the day sleeping during fasting hours.

In nations where Islamic customs are strictly observed, like Saudi Arabia, non-Muslims are asked to refrain from eating, drinking and smoking in public. In some states, expatriates face jail or deportation if they violate the rules.

In more open Muslim nations, the rigors of daytime fasting turn into a carnival of feasting and revelry by night. Even in the Persian Gulf, lavish banquets are normal until the cannons boom at dawn as a signal for fasting to resume.

The feasting is deplored by some clerics.

''Ramadan is a month of devotion to God. Why do we turn it into a season of food?'' an eminent Islamic leader, Sheik Mohammed al-Ghazali of Egypt, asked during an interview published by the Saudi newspaper Asharq Al-Awsat.

Ramadan normally begins when the crescent of the new moon is sighted in the sky. But normally, if no crescent is sighted, as this year, the period starts when the month of Shaaban concludes 30 days. Shaaban ends Sunday this year, though Muslims were to scan the skies for traces of the new moon Sunday night.

The holy month ends when the next crescent moon heralds the three-day feast of Eid al-Fitr.

In some Islamic countries, local religious authorities may still opt to wait for the first sign of the new moon before beginning Ramadan.