Ash-Shaykh Bin Baz, rahimahullah, was asked the following,
“If we intend to fast the six days of Shawwāl consecutively and it so happens that some of the days of Shawwāl fall on a Saturday, should we continue to fast or break our fast?”
“As mentioned previously it is more recommended to fast the six days of Shawwāl consecutively [i.e. right after Ramadan]. However, there is no harm in fasting on Saturday, even if a person were to single it out since the hadith that prohibits singling out Saturday with fasting except in the case of an obligatory fast is [actually] a weak hadith as previously stated; it is not authentic on the Prophet (ﷺ).” End.
Ash-Shaykh Muhammad bin Saalih al-Uthaymeen, rahimahullah, was asked the following,
“Is it permissible to single out Friday with fasting with the intention of fasting the six days of Shawwāl?”
“If a person fasts on Friday because he will be off work on this day and it is difficult for him to fast on other than Friday, then there is no harm for him to single it out [with fasting] as he has not singled out Friday due to it being Friday. But rather, he singled it out only because this is his day off work. However, if he singles it out for no reason at all, then in this case we say either you fast a day before it or a day after it. And the same applies for fasting on Saturday; he shouldn’t single it out with fasting. He should either fast a day before it or a day after it or else he shouldn’t fast [on this day] unless he says [to himself], ‘My only day off from work is on Saturday and I wish to fast the six days of Shawwāl’. In this case we say there is no harm in this as you have not singed it out due it being a Saturday.” End. [Questions from Liqaa al-Baab al-Maftooh]
AbdulFattaah Bin Uthman