Shaykh Ibn Baaz rahimahullaah was asked:
Q: If the day of `Aashooraa (10th of Muharram) were to coincide with a Saturday; is it permissible for us to fast it?
A: There is no harm in a person fasting Saturday unrestrictedly – [whether it was] obligatory or supererogatory [fasts].
As for the narration which mentions the prohibition of fasting on a Saturday, then it is a weak narration that is reported in contradictory forms, all of equal strength [i.e. so it is impossible to give precedence to one of them over another]; and it is contrary to the authentic narrations.
So there is no harm in a Muslim fasting Saturday – whether it was an obligatory fast or a supererogatory fast – even if he did not fast another day along with it.
And the narration which contains the prohibition of fasting Saturday except for that which is obligatory is an unauthentic narration. Rather it is weak, contradictory to that which is more authentic, and contrary to the authentic narrations [in that regard].
See: The official website of Shaykh Ibn Baz rahimahullaah http://www.binbaz.org.sa/node/13750
