Islamic Laws: Fasting » Ascribing lies to Allah and His Prophet
Fatawa of Ayatullah al Uzama Syed Ali al-Husaini Seestani
1757. Fasting is Mustahab on every day of a year except those on which it is haraam or Makrooh to observe a fast...Fasting is also recommended on 27th of Rajab - the day the Prophet (s.a.w.a.) declared his Prophethood.
If a person observes a Mustahab fast, it is not obligatory on him to complete it. In fact, if one of his brethren-in-faith invites him to a meal, it is Mustahab that he accepts the invitation and breaks the fast during the day time even if it may be after Zuhr.