A person who normally fasts Mondays and Thursdays, wants to fast on a Monday in Shawwal intending the Monday fast. Will that be counted as one of the six days of fasting in Shawwal?
Should we fast separately to make up for missed fasts or will the 6 days of Shawwal suffice for 6 days of missed fasts? May Allah reward you with goodness.
You can fast Mondays with the intention of fasting the six days of Shawwal. Similarly, the 13th, 14th and 15th of every month can be included in the six days of Shawwal.
You first need to fast the days you missed due to your menstruation before you start fasting the six days of Shawwal. So you first fast with the intention of compensating the days you missed since the fard is a priority.
Shaikh Ahmad An-Najmi, may Allah have mercy on him, gave a fatwa allowing to fast Shawwal before compensating for missed fasts.
However, the fatawah of Shaikh ibn Baz, ibn Uthaymin and Ubaid Al-Jabiry state that we should start by compensating first because compensating the days of missed fasts is a fard and it is a sort of debt that must be settled, so obligatory acts come before voluntary acts.
We have the hadith:
من صام رمضان ثم أتبعه ستاً من شوال كان كصيام الدهر
He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal. it would be as if he fasted perpetually.
If you fast only twenty-four or twenty-five days of Ramadan, then you actually did not fast the whole month. Therefore to complete the whole month you must compensate for the days you missed.
The hadith is only applicable if you completely fast for the whole month.
It is the arguments or proofs which the Scholars based their fatwa on and I believe this is the strongest opinion.
I know that the fasting of six days will suffice for the three white days of every month. However, you do not take zawab (reward) for both fasting because each type of fasting is an independent worship. Similarly, when we pray the voluntary prayer of Fajr or Dhuhr in the Mosque, it suffices for the prayer we offer when we enter the Mosque.