Fasting six days of the month of Shawwal
What is the extent of the legality for fasting six days of Shawwal (the six 'white' days) and what is their reward?
Is it permissible to combine the intention of fasting them with the intention of making up the missed fast days of Ramadan?
Is it a condition to fast them consecutively?
The merit of fasting six days of Shawwal
Abu Ayub al-Ansari narrated that the Prophet [pbuh] said: "Whoever fasts Ramadan and follows it with six days of Shawwal is as though he has fasted the entire year" [Reported by Muslim in his Sahih]. According to the opinion of many scholars, it is commendable to fast these days. Since Allah rewards each good deed with ten times its value, a Muslim who fasts six days of Shawwal after the fast of Ramadan is considered to have fasted 360 days i.e. an entire year.
Scholars noticed that the relationship of fasting the six days of Shawwal to Ramadan is analogous to that of the voluntary prayers following obligatory prayers and that the relationship of fasting days from Sha'ban is analogous to that of the voluntary prayers preceding the obligatory prayers—they fill the imperfections that may have occurred in an obligatory action. Moreover, they indicate that the fast of Ramadan has been accepted by the will of Allah. This is because among the signs that an act of worship has been accepted, is following it up with another act of worship. Additionally, it shows that a person does not become bored of obedience and thus sets out to fast yet again after breaking his fast on the Day of 'Eid, the Day of Reward.
Manner of fasting
It is not a condition to fast the six days of Shawwal consecutively. For instance, a person can distribute them over the month of Shawwal by fasting on Mondays and Thursdays or on the three white days i.e. the 13th, 14th and 15th of a lunar month.
However, it is best to perform them immediately after the day of 'Eid.
Making up the missed fast days of Ramadan
If possible, it is preferable to make up the missed fast days of Ramadan before fasting the six days of Shawwal. This is due to the hadith that states, "A debt owed to Allah has more right to be fulfilled."
According to Shafi'i scholars, it is possible to combine the intention of making up the missed fast days of Ramadan with the six days of Shawwal. Alternatively, one can fast the six days of Shawwal first and delay making up the missed fast days of Ramadan provided one completes them before the next Ramadan.
The time for making the intention
The intention of fasting the six days of Shawwal can be made any time before noon provided one has not done anything that voids fasting. Such is the case for all voluntary fasts, in contrast to obligatory and make up fasts for which one must make the intention
Allah Almighty knows best.