Some Muslims celebrate the middle of Shaban; they fast on that day and spend that night in prayer. There is a Hadeeth concerning that which is not saheeh; hence, the scholars regarded celebrating this day as an innovation (Bid ‘ah).
Muhammad ‘Abd al-Salam al-Shuqayri said: Imam al-Fatni said in Tadhkirat al-Mawdoo‘at: Among the innovations that have been introduced on “Laylat an-Nusf” (mid-Shaban) is al-Salat al-Alfiyyah, which is one hundred Rakahs in which Soorat al-Ikhlas is recited ten times in each rakah, offered in congregation; they pay more attention to this than to Friday and Eid prayers, although there is no report concerning it, except weak and fabricated reports.
Al-‘Iraqi said: The hadeeth about the prayer on Laylat al-Nisf (mid-Shaban) is false. Ibn al-Jawzi narrated it in al-Mawdoo‘at (which is a compilation of fabricated Ahadeeeth):
Chapter on the Hadeeth, prayer and supplication on Laylat al-Nisf:
The hadeeth, “When the night of ‘nisf Shaban’ (mid-Shaban) comes, spend the night in prayer and fast on that day” was narrated by Ibn Majah from ‘Ali. Muhashiyyah said: (It was also narrated) in al-Zawa’id. Its isnad is da‘eef (weak) because of the weakness of Ibn Abi Basrah, of whom Ahmad and Ibn Ma‘een said: He fabricates hadeeth. End quote.
Praying six Rakahs on Laylat al-Nisf with the intention of warding off calamity, having a long life and being independent of people, and reciting Surah Ya-Seen and offering Dua in between that – there is no doubt that this is something that has been introduced into the religion and is contrary to the Sunnah of the Messenger of Allah (sa).
The commentator on al-Ihya said: Our companions said: It is makrooh to gather on any of the nights mentioned in the Masajid or elsewhere. Al-Najm al-Ghayti said, describing spending the night of al-Nisf min Sh‘ban (mid-Shaban) praying in congregation: That was denounced by most of the scholars of the Hijaz, including Ata and Ibn Abi Mulaykah, the fuqaha of Madinah and the companions of Malik. They said: All of that is an innovation (Bidah) and there is no report to suggest that the Prophet (sa) spent that night in praying in congregation or that his companions did that either. Al-Nawawi said: The prayers of Rajab and Shaban are two reprehensible innovations. (Al-Sunan wal-Mubtadat, p. 144)
Some people use the word al-Shabaniyyah to refer to the last days of Shaban, and say, “These are the days of bidding farewell to food,” and they take advantage of these days to eat a lot before Ramadan begins. Some scholars say that this idea was originally taken from the Christians, who used to do that as their fasting period (Lent) approached.
To sum up, there is no celebration in Shaban and there is no special act of worship to be performed in the middle of it or during the last days of the month. Doing that is an innovation that has been introduced into the religion.
Adapted from http://islam-qa.com/en/ref/154850