What is the ruling on observing a special fast on the 15th of Sha‘baan?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

There is nothing reported in the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam, to encourage the fasting of the 15th of Sha‘baan. Never were the Companions or those who followed them with good conduct reported to have fasted that day. The fasting of the 15th of Sha‘baan is an innovation in religion, and if it was virtuous, then those who were more righteous than us from among those who came before us would have fasted it. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Whoever introduces to this matter of ours (Islam) that which does not belong to it, he will have it rejected.” [Muslim] 

However, there should be no problem with the fasting of the 15th of Sha‘baan if it coincides with a day that the person regularly fasts. This is like when the 15th of Sha‘baan coincides with the person’s habit of fasting every other day, the fasting of Mondays and Thursdays, the fasting of the 13th, 14th, and 15th of every month, or a day that he vowed to fast.

Allah Knows best.