• What is the extent of the legality for fasting six days of Shawwal (the six 'white' days) and what is their reward?

The merit of fasting six days of Shawwal

Abu Ayub al-Ansari narrated that the Prophet (peace and blessings be upon him) said, "Whoever fasts Ramadan and follows it with six days from the month 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 rewards each good deed with ten times its value, a Muslim who fasts six days from 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 six days from 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 God Almighty. 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.

And God Almighty knows best.