Abu Ayub Al-Ansari narrated that the Prophet (peace and blessings be upon him) said: "Whoever fasts Ramadan and follows it with six days of Shawwal is as though he has fasted the entire year" [recorded by Muslim in his Sahih]. According to the opinion of many scholars, it is commendable to fast these days. Since God the Almighty rewards each good deed tenfold, a Muslim who fasts on six days of the month 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 on six days of Shawwal to Ramadan is similar to the relationship of the voluntary prayers which precede and come immediately after an obligatory prayer to that obligatory prayer, just as fasting six days of Sha’ban is similar to the relationship of the voluntary prayers which precede an obligatory prayer — it mends any imperfection that occurred in the obligatory action. It [also] shows that the fast of Ramadan has been accepted by the will of God the Almighty. This is because among the signs that an act of worship has been accepted, is its being followed with another act of worship. Additionally, it shows that a person has not become bored with obedience and thus sets out to fast yet again after breaking the fast for the Day of 'Eid, the Day of Reward.

It is not a condition to fast the six days of Shawwal consecutively. For instance, a person could distribute them over the month of Shawwal by fasting on Mondays and Thursdays or on the three days of the full moon (13th, 14th and 15th of every lunar month) — even though fasting them immediately after ‘Eid is best.

If possible, it is preferable for a Muslim who is able to first fast the unperformed days of Ramadan because of the hadith: "A debt owed to God 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 intention of fasting the six days of Shawwal. Alternatively, it is permissible to fast the six days of Shawwal first and delay fasting the missed fast-days of Ramadan provided one completes them before the next Ramadan.

The intention for fasting the six days of Shawwal can be made at 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 the intention must be made before dawn.

And God the Almighty knows best.