the fourth pillar of islam is fasting. allah prescribes daily fasting for all able, adult muslims during the whole of the month of ramadan, the ninth month of the lunar calendar, beginning with the sighting of the new moon.
exempted from the fast are the very old and the insane. on the physical side, fasting is from first light of dawn until sundown, abstaining from food, drink, and sexual relations. on the moral, behavioral side, one must abstain from lying, malicious gossip, quarrelling and trivial nonsense.
those who are sick, elderly, or on a journey, and women who are menstruating, pregnant, or nursing are permitted to break the fast, but must make up an equal number of days later in the year. if physically unable to do so, they must feed a needy person for each day missed. children begin to fast (and to observe the prayers) from puberty, although many start earlier.
in addition to the fast proper, one is encouraged to read the entire quran. in addition, special prayers, called tarawih, are held in the mosque every night of the month, during which a whole section of the quran (juz') is recited, so that by the end of the month the entire quran has been completed. these are done in remembrance of the fact that the revelation of the quran to prophet muhammad (peace be upon him) was begun during ramadan.
during the last ten days - though the exact day is never known and may not even be the same every year - occurs the night of power (laylat al-qadr). to spend that night in worship is equivalent to a thousand months of worship, i.e. allah's reward for it is very great.
on the first day of the following month, after another new moon has been sighted, a special celebration is made, called 'id al-fitr. a quantity of staple food is donated to the poor (zakat al-fitr), everyone has bathed and put on their best, preferably new, clothes, and communal prayers are held in the early morning, followed by feasting and visiting relatives and friends.
there are other fast days throughout the year. muslims are encouraged to fast six days in shawwal, the month following ramadan, mondays and thursdays, and the ninth and tenth, or tenth and eleventh of muharram, the first month of the year. the tenth day, called ashurah, is also a fast day for the jews (yom kippur), and allah commanded the muslims to fast two days to distinguish themselves from the people of the book.
while fasting per se is encouraged, constant fasting, as well as monasticism, celibacy, and otherwise retreating from the real world, are condemned in islam. fasting on the two festival days, 'id al-fitr and 'id al-adha, the feast of the hajj, is strictly forbidden.