Sawm means to abstain from those things which break the fast, from subh al-sadiq (true dawn) till Maghrib, in obedience to Allah.
8 fasts are obligatory (wajib): (1) Month of Ramadan; (2) Qadha of the month of Ramadan; (3) 3rd day of I’tikaf; (4) 10 days of 'badal al-hadi' in Hajj; (5) Fast of kaffarah becoming obligatory for various reasons, (6) Fast which becomes obligatory on account of nadhr, ‘ahd or qasam; (7) Saum of kaffarah of breaking or leaving fast; (8) Fast which has become qadha from father, becomes obligatory on his eldest son after his death (It is better to include 'mother' also in this rule).
These are the conditions for the validity of the fast:-
1. Sanity; 2.Islam 3. Iman (faith); 4. Remaining free from haidh and nifas for whole day; 5. Not remaining a traveler (musafir) at the time of zawal; 6. Not being in danger of illness by keeping fast. If a traveller returns to his home-town or reaches a place where he intends to stay 10 days, before zawal, and has not done anything which breaks the fast, he must do the 'niyyah' of fasting, and his fast will be valid. Likewise, if he begins his journey after zawal, his fast of that day will remain valid.
If a person is afraid that by keeping fast his illness will increase or he will become sick by fasting, or that the treatment of his disease will become more difficult, he must break his fast. If he does keep fast, it would be null and void, and he, in addition, would be committing a sin. Being baligh is not a condition of validity of fast; if a child keeps fast, it would be all right. But fast is not obligatory on a 'minor' (ghayr-baligh).
The following persons are exempted from fasting:
1. & 2. Old men and women, when owing to their old age and weakness (a) it is not possible for them to keep fast, or (b) it is very difficult to fast.
3. A person who has got a disease in which he remains ever thirsty and (a) it is not possible, or (b) is very difficult, for him to keep fast.
4. The expectant mother who is afraid (a) about her own health or (b) the health of the unborn child.
5. The woman who breast-feeds a child (her own or another woman's) and is afraid that the fast (a) would endanger her health or (b) the health of the child.
All these 5 types of persons are exempted from fasting on the following conditions:-
1. In cases of (b) they will have to pay kaffarah (one 'mudd' (3/4 kilogram) wheat, rice or any such staple food, per day to a poor Ithna-‘ash’ari. It is 'ahwat' to pay l ½ kilogram per day.
2. In cases of (a) they are not required to pay any Kaffarah; but it is highly recommended.
3. Pregnant woman, or the nursing mother will have to fast in qadha after delivery or after weaning the child, as the case may be.
4. The thirsty person will have to fast in qadha, if he is able to do so afterwards.
5. Old people are not required (though recommended) to fast in qadha they regains their strength.