It is desirable to fast on the 15th of Shaban, it is proven by Hadith Sharif. Considering it illegitimate and taunting those who fast on this day, or considering fasting on this day obligatory and taunting those who do not, are all excesses and wrong. Otherwise, it has no separate merit. Therefore, if anyone wants to fast on the 15th of Shaban, they can do so. It is narrated in Sunan Ibn Majah from Hazrat Ali (R.A.):
"The Messenger of Allah (peace and blessings of Allah be upon him) said: When it is the night of the middle of Shaban, then worship at night and fast the next day, because in it, from sunset to dawn, Allah Almighty descends to the lowest heaven and says: Is there anyone seeking forgiveness that I may forgive him! Is there anyone seeking sustenance that I may give him sustenance! Is there anyone sick that I may heal him from sickness! Until dawn breaks."
"Sunan Ibn Majah" states:
"'An Ali ibn Abi Talib (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: When it is the night of the middle of Shaban, then stand up in prayer at night and fast during the day, for Allah descends during it from sunset to the lowest heaven, and says: Is there anyone seeking forgiveness that I may forgive him? Is there anyone seeking provision that I may provide for him? Is there anyone afflicted that I may relieve him? Is there this, is there that, until dawn breaks."
(Abwab Iqamat al-Salawat wa al-Sunnah fiha, Bab ma ja'a fi laylat al-nisf min Shaban, j:2, p:399, r:1388, t:Dar al-Risala al-Alamiya)
Fatawa Alamgiri states:
"(The desirable types of fasting are several) The first is fasting in Muharram, the second is fasting in Rajab, the third is fasting in Shaban, and fasting on Ashura, which is the tenth day of Muharram according to most scholars and companions - may Allah be pleased with them."
(Kitab al-Sawm, al-Bab al-Thalith fima yukrah lil-sa'im wa ma la yukrah jild: 1, p: 202, t: Dar al-Fikr)
Only Allah knows best.
Mufti Sadiq Amin Qasmi