It is Mustahab (i.e. an act of great merit) to observe the six fasts of Shawwal. The observance of these fasts has been emphasised by Rasulullah Sallallahu Alayhi Wa Sallam in several Ahadith. It is the infinite mercy of Allah Ta’ala, that in exchange for minimal effort, he accords maximum reward.
Rasulullah Sallallahu Alayhi Wa Sallam has stated, “Whoever fasted the full month of Ramadhan and then follows it with six fasts of Shawwal is like a person who has fasted the entire year.” (Targhib)
“One who fasted during the month of Ramadhan and further fasted six days in Shawwal, is cleansed from sins like the day he was born.” (Targhib)
§ These fasts may be kept consecutively or at intervals throughout the month of Shawwal.
§ Ladies must first complete their Qadha Ramadhan fasts before commencing with the Shawwal fasts.