All across the Islamic world, from Mauritania to Malaysia, this is the month that turns night into day.

It is Ramadan, one of the four holy months of Islam, the month in which, according to Islamic tradition, the first chapters of the Koran were revealed to the Prophet Mohammed.

Throughout to abstain from food, drink, tobacco and sexual intercouse from first light - determined by the moment when a white thread can be distinguished from a black one, until a cannon tolls the sunset.

As they have for centuries, Moslems respond to the command to fast by doing as little as possible during the daylight hours, sleeping when they can, and feasting much of the night.

In strict Moslem countries like Libya and Kuwait, the cafes, restaurants and juice bars are closed during the day and public eating and drinking are forbidden. A few hotels are allowed to serve food and drink to the few foreigners who visit those countries during Ramadan, but not to Moslems.

Here in relatively liberal, easygoing Egypt, which has a large Christian population, the rules are more relaxed and many snackbars are open, but most of the population either keeps the fast or breaks it only in private. Cairo newspapers reported the arrest of "an Arab" from another country who was caught drunk in public this month.

The Ramadan observance is as old as Islam. The Koran prescribes abstinence during "the month of Ramadan, where in the Koran was sent down as a guidance to men and as evidences of the guidance, and as a criterion of right and wrong. So whoever of you is than at home, let him fast this month.

Because Islam follows the lunar calendar, the dates of Ramadan vary annually and slowly move around the year. The month begins only when the new moon is actually sighted by a believer, even though the moment of the appearance of the new moon can be determined with mathematical certainty in advance.

When Ramadan falls in summer, as it does this year and will for the next few years, the fast is especially difficult because in the latitude of Jeddah or Cairo daylight can last as long as 16 hours and the heat is intense.

The result is that work comes to a near standstill.

In Egypt, public services and the bureaucracy, inefficient at the best of times, seem hardly to function at all.

The minister of state for administration, Dr. Aly Salmy, asked the prime minister to order government workers and employees of the state-owned industries to put in a full day during this Ramadan to cut the economic burden on the country, but there is no sign that his plea was heeded. The working day, for those who bother to show up, is from about 10 a.m. to 2 p.m.

Life begins at sunset, when the radio broadcasts prayers and the cannon shot notifies the faithfuly that the day's fast has ended.

Families and friends gather for the traditional iftar , or breakfast, a feast washed down with apricot nectar and followed by cakes and sherbets.

In Egypt, much of the population eats beyond its means during Ramadan and the government lays in special supplies of subsidized commodities like sugar so there will be enough for all at reasonable prices. Workers have come to expect Ramadan bonuses to help them buy extra fancy foods this month.

After the meal, some shops reopen and some government officials and businessmen go back to work, but most people party until late into the night - even the children, who run through the streets singing and waving lanterns.

Ramadan is the month of two great events in Islamic history - the revelation of the Koran and the battle of Badr.

In the year 624, about 300 followers of the prophet, who by then had moved from Mecca where he was scorned to Medina where he was welcomed, surprised a reinforced Meccan caravan on its way home from Syria. The victory they scored over 1,000 Meccans, modern scholars say, laid the foundation for Mohammed's temporal power and began the train of propagation of Islam by armed conquest.

So revered is that event by Moslems even today that the Arab attack on the Israelis in 1973 was code named "Operation Badr," because it occured during Ramadan.

As he does every year, the grand sheikh of Cairo's Al Azhar mosque and university, the citadel of Islamic learning, took the occasion of Ramadan to address a message to all Moslems.

He exhorted the faithfully to uphold the principles of Islam, "first of all kindness," "which benefit only the enemies of Islam."

The grand sheikh, Dr. Abdel Halim Mahomoud, called for solidarity with "Moslem minorities in the Philppines, western Somalia, Ethiopia and elsewhere," and asked for a renewal of solidarity that would enable Arab Moslems to "regain the usurped land and liberate Jersualem, the holy place, blessed by Allah."