Regarding singling out saturday for fasting if Ashura falls on it, Shaykh Al-Uthaymeen (may Allah have mercy upon him) said:
And know that fasting on a Saturday has it specific cases:
- First situation: That it be an obligatory fast, like that of Ramadhan, or making up a fast of Ramadhan, or fasting as a kafaaarah (expiation), or due to not being able to slaughter on Hajj Tamatu, or the likes of this, then there is no problem in this as long as he does not specify this day as believing it to have a distinctive feature.
- Second situation: That he fasts the Friday before it then there is no problem in this, because the Prophet (salallahu alayhi wa salam) said to one of the mothers of the believers and she had fasted on the day of Friday: He said; “Did you fast yesterday?” She said, “No.” He said: “Do you intend to fast tomorrow?” She said: “No.” He said: “Then break your fast.” so his statement “will you fast tomorrow?” proves the permissibility of fasting Saturday if Friday was fasted also.
- Third situation: The day of Saturday falls on a legislated day like the Ayaam al-Beeydh (white days 13,14,15 Islamic calendar) or the day of Arafah, or the day of Ashuwra, or the six days of Shawaal for the one who fasts Ramadhan, or the 9th of Dhul Hijjah then there is no problem, because the fast was not fasted because it is a Saturday but because it is from the days that fasting has been legislated.
- Fourth situation: It falls on a day you habitually fast like that fast of fasting a day and not fasting the next so your fast falls on a Saturday the there is no problem in this as the Prophet (salallahu alayhi wa salam) said when he prohibited fasting before the start of Ramadhan by a day or two “except for a man who normally fasts then let him fast”. And this is a similar situation.
- Fifth situation: That you single out Saturday for fasting voluntary fasts then this is the place of prohibition of fasting on a Saturday if the hadeeth of the prohibition of fasting on a Saturday is authentic.
End of quote
“Majmoo al-Fatawaa Wa Rasaail – Sheikh al-Uthaymeen rahimahullah” (20/57)
Translated by Ahmed Iqbal (On behalf of MSL)