Is Fasting on the 15th of Sha`ban Recommended?
Question

Is it permissible, even after finding out that a hadith is da’if (weak), to follow it, by way of doing righteous deeds? The hadith says: “When it is halfway through Sha’baan, spend that night in prayer and fast on that day.” The fast is observed as a voluntary act of devotion to Allah, as is spending that night in prayer (qiyaam al-layl).
Praise be to Allah.15th of Sha`ban: Any recommended worship?
What is narrated concerning the virtue of praying, fasting and worshipping on the fifteenth of Sha'ban (al-Nisf min Sha’ban) does not come under the heading of da’if (weak), rather it comes under the heading of mawdu’ (fabricated) and batil (false). So it is not permissible to follow it or to act upon it, whether that is in doing righteous deeds or otherwise.
A number of scholars ruled that the reports concerning that were false, such as Ibn al-Jawzi in his book al-Mawdu’at, 2/440-445; Ibn Qayyim al-Jawziyyah in al-Manar al-Munif, no. 174- 177; Abu Shamah al-Shaafi’i in al-Ba’ith ‘ala Inkar al-Bida’ wa’l-Hawadith, 124-137; al-'Iraqi in Takhrij Ihya ‘Uloom il-Din, no. 582. Shaykh al-Islam [Ibn Taymiyah] narrated that there was consensus on the fact that they are false, in Majmu’ al-Fatawa, 28/138.
Shaykh Ibn Baz (may Allah have mercy on him) said in Hukm al-Ihtifal bi Laylat al-Nisf min Sha’ban (Ruling on celebrating the fifteenth of Sha'ban):
“Celebrating the night of the fifteenth of Sha'ban (Laylat al-Nisf min Sha’ban) by praying etc, or singling out this day for fasting, is a reprehensible bid’ah (innovation) according to the majority of scholars, and there is no basis for this in Shari’ah.”
And he (may Allah have mercy on him) said:
“There is no sahih hadith concerning the night of the fifteenth of Sha'ban (Laylat al-Nisf min Sha’ban). All the ahadith that have been narrated concerning that are mawdu’ (fabricated) and da’if (weak), and have no basis. There is nothing special about this night, and no recitation of Quran or prayer, whether alone or in congregation, is specified for this night. What some of the scholars have said about it being special is a weak opinion. It is not permissible to single it out for any special actions. This is the correct view. And Allah is the Source of strength.” (Fatawa Islamiyyah, 4/511)
Can Muslims act upon weak hadiths about good deeds?
Even if we assume that the hadith is da’if (weak) and not mawdu’ (fabricated), the correct scholarly view is that weak ahadith should not be followed at all, even if they speak of righteous deeds or of targhib and tarhib (promises and warnings). The sahih reports are sufficient and the Muslim has no need to follow the da’if reports. Nothing is known in Islam to suggest that this night or day is special, either from the Prophet (peace and blessings of Allah be upon him) or from his Companions.
The scholar Ahmad Shakir said: “There is no difference between rulings or righteous deeds; we do not take any of them from da’if reports, rather no one has the right to use any report as evidence unless it is proven to be soundly narrated from the Messenger of Allah (peace and blessings of Allah be upon him) in a sahih or hasan hadith.” (Al-Ba'ith al-Hathith, 1/278)
And Allah knows best.