Is there anything affirmed in the Revealed Law that indicates that it is recommended to supplicate (duʿāʾ) and fast the middle day of Shaʿbān, or fast the day following the night of the Isrāʾ wa Al-Miʿrāj on the 27th of Rajab?
Answer (from Sheikh Wahbah Az-Zuḥaylī):
There is nothing affirmed in the Revealed Law that indicates that one should spend the night of the middle day of Shaʿbān in group supplication, nor is there any specified prayer (ṣalāh) therein. Likewise, there is nothing affirmed regarding it being recommended to fast the middle day of Shaʿbān, or the day of the Isrāʾ wa al-Miʿrāj due to the difference of opinion amongst the ʿulamāʾ regarding its exact date. Amongst the things that are rejected is the supplication that is frequently used on the middle day of Shaʿbān, for it contains that which transgresses the bounds of the Revealed Law when, in the supplication, it is said, “Allah erases whatever He wills or endorses it. The Master Copy of the Book is with Him” [Sūrat Ar-Raʿd 13:39], because the foundations of things are never erased or reaffirmed and thus differ with what is written on the Preserved Tablet (al-Lawḥ al-Maḥfūẓ).
The ḥadīth that has been related by Ibn Mājah on the authority of ʿAlī, may Allah be pleased with him, in which the Prophet, may Allah bless him and grant him peace, said, “If it is the night of the middle day of Shaʿbān, then stand during its night and fast during its day, for indeed Allah, Blessed and Exalted, descends therein upon the setting of the sun to the lowest sky, and says, “Is there anyone seeking forgiveness that I can forgive? Is there anyone seeking provision that I can provide for? Is there anyone suffering that I can relieve? Is there anyone such-and-such? Is there anyone such-and-such?” until the crack of dawn” is not authentic (ṣaḥīḥ).
The only thing that has been authentically narrated from the Prophet, may Allah bless him and grant him peace, is regarding the virtue of the entire month of Shaʿbān, without distinguishing one night over another, and this is why he would fast a lot during this month. Also, it is unlawful to fast in the second half of Shaʿbān unless one has fasts to make up or those days coincide with one’s normal fasts.
[Translated from Fatāwā Muʿāṣirah (Damascus: Dār Al-Fikr, 1427/2006), p.36-37]