It is Mustahab and highly rewarding to observe the six fasts of Shawwaal. 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 Ramadaan and then follows it with six fasts of Shawwaal is like a person who has fasted the entire year.” (Targhib)
“One who fasted during the month of Ramadaan and further fasted six days in Shawwaal, is cleansed from sins like the day he was born.” (Targhib)
These fasts may be kept consecutively or at intervals during the month of Shawwaal.
Women with Qadha fasts MUST keep their Qadha fasts first before keeping the Shawwaal fasts.