[A question asked by a fellow Muslim, and His Eminence replied on 9/2/1415 A.H., no. Sh/1179]
To His Eminence, Shaykh `Abdul-`Aziz ibn `Abdullah ibn Baz, the Grand Mufty (Islamic scholar qualified to issue legal opinions) of Saudi Arabia, may Allah protect him.
As-salamu `alaykum warahmatullah wabarakatuh (May Allah’s Peace, Mercy, and Blessings be upon you).
As you know, this year the 9th of Muharram was on Saturday, and the 10th was on Sunday, according to the Makkan calendar. Acting upon the Hadith that states: If I live until next year, I will perform Sawm (Fasting) on the 9th and 10th (of Muharram)  I performed Sawm on Saturday and Sunday. However, one of my Muslim brothers objected to my Sawm on Saturday, and told me that it is forbidden to perform a voluntary Sawm on Saturdays, based on a Hadith, which he said its meaning and not its text. Due to my desire for clarifying this matter, and acting upon the Words of Allah (Exalted be He): So ask the people of the Scripture, if you do not know.  I would like Your Eminence to clarify this ambiguity, mentioning the Hadith and the degree of its authenticity, and advise me on this matter. May Allah protect you. 
A: Wa `alaykum as-salam warahmatullah wabarakatuh (May Allah’s Peace, Mercy, and Blessings be upon you too).
The Hadith he mentioned is well-known and included in: “Bulugh Al-Maram”, in the Chapter on Sawm. However, it is a Hadith Da`if (a Hadith that fails to reach the status of Hasan, due to a weakness in the chain of narration or one of the narrators) and Shadh (a Hadith narrated by a trustworthy narrator, not in line with the narration of other trustworthy narrators in terms of wording, chain of narrators, or both), and contradicts the Hadith Sahih (Hadith that have been transmitted by people known for their uprightness and exactitude; free from eccentricity and blemish) such as the next one in which the Prophet (peace be upon him) said: Do not perform Sawm on Fridays, unless you perform Sawm on the day before it or the day after it.  It is known that the day that follows Friday is Saturday. Also, the Hadith, which is mentioned in the Two Sahih Books of Hadith (i.e. Al-Bukhari and Muslim): The Prophet (peace be upon him) used to perform Sawm on Saturdays and Sundays, saying: “They are feast days for the Mushriks (those who associate others with Allah in His Divinity or worship), and I like to do the opposite of them. 
There are many Hadiths that carry the same meaning, and they all show the permissibility of voluntary Sawm on Saturday.
May Allah grant us success. As-salamu `alaykum warahmatullah wabarakatuh.
The Grand Mufty of the Kingdom of Saudi Arabia
and Chairman of the Council of Senior Scholars
and the Departments of Scholarly Research and Ifta’
 Narrated by Muslim, Book on fasting, Chapter on which day the fast is observed in Muharram, no. 1134
 Surah Al-Nahl, 16: 43
 A question asked by a fellow Muslim, and His Eminence replied on 9/2/1415 A.H., no. Sh/1179.
 Al-Bukhari, Sahih, Book on fasting, no. 1985; Al-Tirmidhy, Sunan, Book on fasting, no. 743; Abu Dawud, Sunan, Book on fasting, no. 2420; and Ibn Majah, Sunan, Book on fasting, no. 1723
 Ahmad ibn Hanbal, Musnad, vol. 6, p. 324