Reference: al-Bid’u wal-Muhdathaat wa maa laa asla lahu – Page 614
Noorun alad-Darb Fataawa Shaykh Saalih Ibn Fowzaan – Volume 1, Page 87
Question: Is standing the night of the 15th of Sha’baan in prayer and fasting during it’s day legislated?
Response: Nothing firm and reliable has been established on the authority of the Prophet ﷺ that he stood in prayer in the night and fasted during the day of the 15th of Sha’baan. So the night of the 15th of Sha’baan is like any other night, and if someone is a regular worshipper during other nights, then he may stand the night in prayer on this night without assuming anything special [because of it being the night of the 15th of Sha’baan]. This is because specifying a time for any act of worship requires a authentic proof, so if there is no authentic proof then the act is regarded as an innovation and all innovations are misguidance. Likewise, regarding specifically fasting during the 15th day of Sha’baan, then no [authentic] proof has been established on the authority of the Prophet ﷺ to indicate the legislation of fasting on that particular day.
As for that which is mentioned from the ahaadeeth regarding this subject, then all of it is weak as the people of knowledge have indicated. However, whoever has the habit of fasting the 13th, 14th and 15th (of every month), then he can continue and fast during Sha’baan as he fasts during the other months, without assuming anything special about the 15th of Sha’baan. Also, the Prophet ﷺ used to increase in fasting during this month (Sha’baan), however, he did not specifythe 15th day, rather proceeded as per norm.