What is the ruling on fasting six days of Shawwal? Is it wajib (obligatory)?
Praise be to Allah.
Fasting six days of Shawwal after the obligatory fast of Ramadan is Sunnah Mustahabbah, not wajib. It is recommended for the Muslim to fast six days of Shawwal, and in this there is great virtue and an immense reward. Whoever fasts these six days will have recorded for him a reward as if he had fasted a whole year, as was reported in a sahih hadith from the Prophet (peace and blessings of Allah be upon him). Abu Ayyub (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings of Allah be upon him) said:
“Whoever fasts Ramadan and follows it with six days of Shawwal, it will be as if he fasted for a lifetime.”Narrated by Muslim, Abu Dawud, At-Tirmidhi, An-Nisa’i and Ibn Majah
The Prophet (peace and blessings of Allah be upon him) explained this when he said: “Whoever fasts for six days after (‘Eid) Al-Fitr has completed the year: (whoever does a good deed (hasanah) will have ten hasanah like it).” According to another report: “Allah has made for each hasanah ten like it, so a month is like fasting ten months, and fasting six days completes the year.” [An-Nisa’i and Ibn Majah. See also Sahih at-Targhib wa’t-Tarhib, 1/421). It was also narrated by Ibn Khuzaymah with the wording: “Fasting for the month of Ramadan brings the reward of ten like it, and fasting for six days brings the reward of two months, and that is the fasting of the whole year.”
The Hanbali and Shafi’i fuqaha’ explained that fasting six days of Shawwal after fasting Ramadan makes it as if one has fasted for an entire year of obligatory fasts, because the multiplication of the reward applies even to nafil fasts, because each hasanah brings the reward of ten like it.
Another of the important benefits of fasting six days of Shawwal is that is makes up for any shortfall in a person’s obligatory Ramadan fasts, because no one is free of shortcomings or sins that have a negative effect on his fasting. On the Day of Resurrection, some of his nafil deeds will be taken to make up the shortcomings in his obligatory deeds, as the Prophet (peace and blessings of Allah be upon him) said:
“The first thing for which people will be brought to account on the Day of Resurrection will be their salah (prayer). Our Lord, may He be glorified and exalted, will say to His angels – although He knows best – ‘Look at the salah of My slave, whether it is complete or incomplete.’ If it is perfect, it will be recorded as perfect, and if something is lacking, He will say, ‘Look and see whether My slave did any voluntary (nafil) prayers.’ If he did some voluntary prayers, [Allah] will say, Complete the obligatory actions of My slave from his voluntary actions.’ Then all his actions will be dealt with in a similar manner.”Narrated by Abu Dawud
And Allah knows best. 
When should a Muslim start fasting six days of Shawwal? When can I start fasting six days of Shawwal, since we have annual leave right now?
Praise be to Allah.
You can start fasting six days of Shawwal from the second day of Shawwal, because it is haram to fast on the day of ‘Eid. You can fast the six days at any time during Shawwal, although the best of good deeds are those which are done soonest.
The standing committee received the following question:
Should fasting the six days be done immediately after Ramadan, following the day of ‘Eid or is it permissible to do it a few days after ‘Eid in the month of Shawwal or not?
They replied as follows:
These days do not have to be fasted immediately after ‘Eid Al-Fitr; it is permissible to start fasting them one or more days after ‘Eid, and they may be done consecutively or separately during the month of Shawwal, according to what is easier for a person. There is plenty of room for manoeuvre in this matter, and this is not obligatory, it is Sunnah.
And Allah is the Source of strength. May Allah bless our Prophet Muhammad and his family and companions and grant them peace. 
Do the six days of Shawwal have to be fasted consecutively?
 With regard to the six days of Shawwal after Ramadan, is it a condition that they should be fasted consecutively, or can I separate them? I want to fast them in three sessions, on the two days of the weekend.
Praise be to Allah.
It is not a necessary condition that they should be fasted consecutively. If you fast them separately or consecutively, it is OK. The sooner you do them, the better, because Allah says (interpretation of the meanings):
“So compete in good deeds.”Al-Qur’an 5:48
“And march forth in the way (which leads to) forgiveness from your Lord.”Al-Qur’an 3:133
“[Musa – peace be upon him – said:] … and I hastened to You, O my Lord, that You might be pleased.”Al-Qur’an 20:84
And (you should hasten to fast these six days) because delaying may cause problems. This is the view of the Shafi’is and some of the Hanbalis, but it is OK if you do not hasten it and you delay it until the middle or end of the month.
An-Nawawi (may Allah have mercy on him) said:
“Our companions said: it is mustahabb to fast six days of Shawwal. Because of this hadith they said: it is mustahabb to fast these days consecutively at the beginning of Shawwal, but if one separates them or delays them until after Shawwal, this is permissible, because he will still be following the general guidelines of the hadith. We have no dispute regarding this matter, and this was also the view of Ahmad and Dawud.”Al-Majmu’ Sharh al-Muhadhdhab
Furthermore “… although the majority of the scholars didn’t obligate the six days of Shawwal to have to be immediate, or six consecutive days, it is easier and safer to do it that way and you will avoid any differences between the scholars that way. However, the matter is flexible, so whenever you can.” 
Can a person start fasting six days of Shawwal when he still has days to make up from Ramadan?
Praise be to Allah.
The precise rewards for the deeds which people do for the sake of Allah is something which is known only to Allah. If a person seeks the reward from Allah and strives to obey Him, his reward will not be lost, as Allah says (interpretation of the meaning):
“We shall not make the reward of anyone who does his (righteous) deeds in the most perfect manner to be lost.”Al-Qur’an 18:30
If someone has missed some of the days of Ramadan, he should fast them first, then fast six days of Shawwal, because he cannot follow the fast of Ramadan with six days of Shawwal unless he has completed his Ramadhan fast. 
- if there are too many and it will push you over and you will miss Shawwal, don’t worry. As soon as you are finished then fast the “six days of Shawwal” whatever the date is and you will get the reward for it, insha’Allah, because of the necessity. This was the fatwa of Shaykh Ibn Al-‘Uthaymin (rahimahullah); and
- if you think you will find it difficult to make up all your obligatory fasts now, especially because of the length of day and heat, then leave them for later in the year and fast the six days of Shawwal now. This was performed by ‘A’ishah (radhiallahu ‘anha) and despite the discussion over her action, her example can be used as an evidence for anyone who cannot fast the obligatory qadha’ now, and will put the voluntary fasts before the obligatory.
Does one have to fast six days of Shawwal every year?
If someone fasts six days of Shawwal, then one year he gets sick or something prevents him from fasting, or he is too lazy to fast, will there be any sin on him? We have heard that if a person fasts these six days one year, he must never give up this practice thereafter.
Praise be to Allah.
Fasting six days of Shawwal after the day of ‘Eid is Sunnah. It is not wajib (obligatory) on the person who does this once or more often to continue doing it. He is not guilty of a sin if he does not fast these days.
And Allah is the Source of strength. May Allah bless our Prophet Muhammad and grant him peace. 
Can one make a dual intention when fasting (i.e. for making up their obligatory fasts and, at the same time, the six days of Shawwal)?
There is a temptation (and I guess a belief) that one can start making up their obligatory fasts now and at the same time make a dual intention for the six days of Shawwal too, on the same single day, to try and kill two birds with one stone. This is incorrect, and is a minority opinion.
Although this is something permissible for general matters such as combining the intention for tahiyyat al-masjid and the sunnah of wudhu’ and the two sunnah before dhuhr for example, or combining the intention for the voluntary fast of Monday and the fast for the middle three bright days of the lunar month etc., it is not correct to do it in *this* qadha’/Shawwal case because you have (a) a specific obligation to make up and (b) a specific voluntary fast to perform, which require separate intentions and focus, quite apart from the fact that the hadith concerning the six days of Shawwal makes it clear that they follow the fasts of Ramadan, and that would not be possible if the fast itself is both an owing Ramadan fast *and* a Shawwal fast at the same time.
Thus, these must be done separately. As I have mentioned above, it is best that one does the obligatory qadha’ missed fasts first, and then the six days of Shawwal separately afterwards. Or, they do the six days of Shawwal now and then do the qadha’ afterwards. But not together on the same day with the same intention.
This is the position of the vast majority of the scholars. Wallahu a’lam.