At exactly 3:58 next Saturday morning, Washington time, Muslims around the world will begin their annual month of prayers and fasting, called Ramadan.

Every day during Ramadan, from predawn until sunset, "every sane adult Muslim, male or female," according to instructions issued by the Islamic Center here, will refrain from eating drinking and sexual acts.

Islamic law permits some exemptions, for example, for persons who are sick, or who are obligated to travel a distance of at least 50 miles. Pregnantwomen and nursing mothers are also exempt, but they like others who have valid reasons for not fasting must make up the fast when the reason for the exemption is past.

Persons who are incapaciated by age or by illness or who are engaged in hard labor are permanently exempt, they are expected to observe Ramadan by contributing to the poor the equivalent of the cost of a day's rations for each day of fasting they miss.

Any day during Ramadan in which the fast is broken must be made up at a later date: violations of the restrictions against sexual intercourse must be atoned for by fasting for 60 consecutive days or providing a days food for 60 poor persons.

In addition to the regulations for fasting Muslims are enjoined to be especially generous during Ramadan. A suggested goal is the contribution of 2 1/2 percent of the individual's savings for the year.

While greater generosity is encouraged, the faithful are required to contribute an amount equivalent to the cost of feeding an average adult for one day - an item that the Islamic center estimates to be "at least $3 per person."

Ramadan ends with the feast day of Idul Fitr which this year falls on Sept. 4. On that day, devout Moslems will travel to their mosque to pray before enjoying a feast with their families and friends.