In answering your question let me cite here my earlier answers on this issue:

There are no prescribed rituals of worship to be performed on the 15th of Sha`ban. If there had been any such rituals to be performed, then it would have been stated clearly in the authentic traditions.

Having said this, we cannot stop people from performing nawafil any time; for nawafil are simply optional prayers; no one can stop anyone from performing them any time except those times when prayers are forbidden.

As far as the Sunnah of the Prophet (peace and blessings be upon him) is concerned, we are told by `A’ishah, the Mother of the believers (may Allah be please with her) that the Prophet “used to fast most of the days of Sha`ban, but he stopped fasting when he got close to the month of Ramadan.”

Based on this, it is considered highly recommended to fast in the early part of Sha`ban and stop fasting the last few days of the month.

The Prophet wanted to teach us an important wisdom that we should not join Sha`aban and Ramadan in fasting, rather we should keep the Ramadan fast totally separate from that of Sha`ban. Sha`ban fasting is only for preparation and warming up for the fasting in Ramadan, so we should not confuse the two fasts.

Almighty Allah knows best.