The days other than Ramadan on which it is Mustahab to observe Sawm

A: Observing Sawm in Ramadan is one of the five pillars of Islam. The textual proof of its obligation is the Qur'an, Sunnah, and Ijma` (consensus of scholars).In the Qur'an, Allah (Exalted be He) says: O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqun (the pious - see V.2:2)... And: The month of Ramadan in which was revealed the Qur’ân, a guidance for mankind and clear proofs for the guidance and the criterion (between right and wrong). So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Saum (fasts) that month, and whoever is ill or on a journey, the same number [of days which one did not observe Saum (fasts) must be made up] from other days. (Part No. 9; Page No. 301) In the Sunnah, the Prophet (peace be upon him) said: Islam is based on five (pillars)... And he mentioned Sawm of Ramadan as one of them. Also, on the authority of Talhah Ibn `Ubaydullah who narrated that: A Bedouin with unkempt hair came to the Prophet (peace be upon him) and said: “O Messenger of Allah! Tell me what Allah has enjoined on me as regards Sawm." The Prophet (peace be upon him) said, “The month of Ramadan." He asked: "Do I have to do more than that?" He (peace be upon him) said: "No, unless you wish to perform voluntary (Sawm)." (Agreed upon by Al-Bukhari and Muslim)There is consensus among Muslims that observing Sawm in Ramadan is obligatory and is one of the pillars of Islam. The days on which it is Mustahab to observe Sawm include the six days of Shawwal for those who completed their Sawm of Ramadan; the Day of `Ashura' (10th of Muharram) and one day before or after it; the Day of `Arafah (9th of Dhul-Hijjah) for non-pilgrims; three days each month and it is better to observe them on the White Days (13th, 14th, and 15th of every Hijri month); Mondays and Thursdays; every other day; and during Muharram.The days on which it is prohibited to observe Sawm include the two `Eids; the Days of Tashriq (the 11th, 12th, 13th of Dhul-Hijjah) except for a Mutamatti` (pilgrim performing `Umrah during the months of Hajj, followed by Hajj in the same year with a break in between) or a Qarin (pilgrim combining Hajj and `Umrah without a break in between) if they cannot find a Hady (sacrificial animal offered by pilgrims); and Friday if it is singled out for voluntary Sawm, (Part No. 9; Page No. 302) but there is no harm if one fasts a day before or after it.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.