Some of the scholars have used the following narration to show that if a person were to combine between fasting the six days of Shawwāl and fasting Mondays and Thursdays, he will attain both rewards with one intention since what is mainly intended for fasting on Mondays and Thursdays is that our deeds be raised up while we are fasting.
عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ “ تُعْرَضُ الأَعْمَالُ يَوْمَ الاِثْنَيْنِ وَالْخَمِيسِ فَأُحِبُّ أَنْ يُعْرَضَ عَمَلِي وَأَنَا صَائِمٌ ”
Abu Hurairah narrated that the Messenger of Allah ﷺ said,
‘Deeds are presented on Monday and Thursday, and I love that my deeds be presented while I am fasting'” [Reported by at-Tirimidhi and an-Nasā’i. Graded Saheeh by Shaykh Albani rahimahullah]
Ash-Shaykh Muhammad bin Saalih al-Uthaymeen, rahimahullah, said,
“…If the fast for the six days coincides with Monday or Thursday, then he shall attain the reward of his intention for the six days of Shawwāl along with the reward of [fasting] Monday or Thursday due to the statement of the Prophet (ﷺ),
‘Verily actions are by intentions, and for every person is what he intended.’
As a matter of fact, some scholars hold that it is permissible to combine between fasting days owed from Ramadān with fasting Mondays and Thursdays.
Shaykh Muhammad bin Saalih al-Uthaymeen rahimahullah said,
“… If an individual fasted Thursday and Monday with the intention of making days owed upon him from Ramadān, then we hope that he will attain the reward for the missed days and the reward for fasting these two days, since what is intended by these days is that a person is fasting during them.” End.
However, what is more preferable as been mentioned by other scholars is to do each act separately since in reality some actions would be overlapped by the entry of another.
Ash-Shaykh Saalih al-Fawzaan, hafidahullah, was asked the following question,
“What is the ruling on combining between fasting the white days of the month, the [regular] three days of a month, or Mondays and Thursdays alongside the six days of Shawwāl? Would the person attain a reward for each one?”
“ … As for the statement of the questioner, ‘would he attain the reward for fasting three days of a month and fasting the white days of the month [i.e., the 13th, 14th and 15th of each lunar month], then the Muslim shouldn’t limit his deeds such that the reward for each action be reduced. Rather, he should fast the six days of Shawwāl, fast the white days of the month, and fast three days of the month [separately], which would result in him having much reward. Therefore, he shouldn’t limit these acts to just one intention.” End. [Reference]
The Lajnah ad-Daa’imah was asked the following question,
“Does fasting the white days of the month suffice for fasting Mondays and Thursdays?”
“Fasting the white days of every [lunar] month; 13th, 14th and 15th, and fasting Mondays and Thursdays of every week are all considered as separate acts of worship that have been legislated. Therefore, if you were to fast some of them, then you will attain its reward.” End. [Reference]
AbdulFattaah Bin Uthman