There is only one blessed night, the night of Qadr, mentioned in the Qur’an. The Prophet (SAW) recommended nafl Ibadah in the last 10 nights of Ramadan, one of which may be the night of Qadr. This is the night when the Qur’an was revealed and when angels bring Allah’s Decrees.
The Prophet (SAW) visited the graveyard of Al-Baqi one night and said a prayer for those who died as believers. There are no authentic reports which tell us exactly which night this was.
The Bidd’ah of 15 Sha’ban was propagated by scholars like Abdul Qadir Jilani. Primarily, this has gained fan following because of Sufism’s hold in the Subcontinent. Sufis are very comfortable with the idea of Bidd’ah and have either created many new rituals in the name of Islam or ‘Islamised’ the rituals of other religions.
Therefore, 15th of Sha’ban is not significant in anyway. As Muslims, we must stay away from this Bidd’ah and ensure that we tell our family and friends that all Bidd’ah are grave errors and should be rejected.