Definately a da'ef report. No saheeh hadeeths have been reported regarding the middle of sha'baan.
Shaykh ibn Jibreen [from islamqa] wrote:
There is no saheeh marfoo’ report that speaks of the virtue of the middle of Sha’baan that may be followed, not even in the chapters on al-Fadaa’il (chapters on virtues in books of hadeeth etc.). Some maqtoo’ reports (reports whose isnaads do not go back further than the Taabi’een) have been narrated from some of the Taabi’een, and there are some ahaadeeth, the best of which are mawdoo’ (fabricated) or da’eef jiddan (very weak). These reports became very well known in some countries which were overwhelmed by ignorance; these reports suggest that people’s lifespans are written on that day or that it is decided on that day who is to die in the coming year. On this basis, it is not prescribed to spend this night in prayer or to fast on this day, or to single it out for certain acts of worship. One should not be deceived by the large numbers of ignorant people who do these things. And Allaah knows best.
The majority of the scholars say it's bid'ah.
Shaykb bin baaz (May Allaah have mercy on him said (exerpts):
What the majority of scholars say is that celebrating this occasion is bid’ah, and that the ahaadeeth concerning the virtues of this occasion are all da’eef (weak), and some of them are mawdoo’ (fabricated) Among those who pointed this out was al-Haafiz Ibn Rajab, in his book Lataa’if al-Ma’aarif, and others. The da’eef ahaadeeth concerning acts of worship can only be acted upon in the case of acts of worship which are proven by saheeh evidence. There is no saheeh basis for celebrating the middle of Sha’baan, so we cannot follow the da’eef ahaadeeth either.
This important principle was mentioned by Imaam Abu’l-‘Abbaas Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him).
The [brief] evidences quoted from the fatwa:
quote
Al-Haafiz Ibn Rajab (may Allaah have mercy on him) said in his book Lataa’if al-Ma’aarif concerning this matter – after previously discussing it –
“Laylat al-Nusf min Sha’baan (the middle of Sha’baan) was venerated by the Taabi’een among the people of al-Shaam, such as Khaalid ibn Mi’daan, Makhool, Luqmaan ibn ‘Aamir and others, who used to strive in worship on this night. The people took the idea of the virtue of this night and of venerating it from them. It was said that they heard of Israa’eeli reports (reports from Jewish sources) concerning that. Most of the scholars of the Hijaaz denounced that, including ‘Ataa’ and Ibn Abi Maleekah. ‘Abd al-Rahmaan ibn Zayd ibn Aslam narrated that view from the fuqahaa’ of Madeenah, and this was the view of the companions of Maalik and others. They said: this is all bid’ah… No comment from Imaam Ahmad concerning Laylat al-Nusf min Sha’baan is known of… Concerning spending the night of the middle of Sha’baan in prayer, there is no sound report from the Prophet (peace and blessings of Allaah be upon him) or from his companions…”
This is what was said by al-Haafiz Ibn Rajab (may Allaah be pleased with him). He clearly states that there is no sound report from the Prophet (peace and blessings of Allaah be upon him) or from his companions (may Allaah be pleased with them) about Laylat al-Nusf min Sha’baan (the middle of Sha’baan).
Al-‘Allaamah al-Shawkaani (may Allaah have mercy on him) said in al-Fawaa’id al-Majmoo’ah:
“The hadeeth: ‘O ‘Ali, whoever prays one hundred rak’ahs on Laylat al-Nusf min Sha’baan, reciting in each rak’ah the Opening of the Book [Soorat al-Faatihah] and Qul Huwa Allaahu Ahad ten times, Allaah will meet all his needs…’ This is mawdoo’ (fabricated) [i.e., it is falsely attributed to the Prophet (peace and blessings of Allaah be upon him]. Its wording clearly states the reward that the person who does this will attain, and no man who has any common sense can doubt that this is fabricated. Also, the men of its isnaad are majhool (unknown). It was also narrated via another isnaad, all of which is mawdoo’ (fabricated) and all of whose narrators are majhool (unknown).
In al-Mukhtasar he said:
The hadeeth about the salaah for the middle of Sha’baan is false, and the hadeeth of ‘Ali narrated by Ibn Hibbaan – “ When it is the night of the middle of Sha’baan, spend that night in prayer and fast that day” – is da’eef (weak).
In al-La’aali’ he said,
“One hundred rak’ahs in the middle of Sha’baan, reciting (Soorat) al-Ikhaas ten times in each… (this is) mawdoo’ (fabricated), and all its narrators in its three isnaads are majhool (unknown) and da’eef (weak). He said: and twelve rak’ahs, reciting al-Ikhlaas thirty times in each, this is mawdoo’; and fourteen (rak’ahs), this is mawdoo’.
A group of fuqahaa’ were deceived by this hadeeth, such as the author of al-Ihyaa’ and others, as were some of the mufassireen. The prayer of this night – the middle of Sha’baan – was described in different ways, all of which are false and fabricated.”
Al-Haafiz al-‘Iraaqi said:
“The hadeeth about the prayer during the night of the middle of Sha’baan is fabricated and is falsely attributed to the Messenger of Allaah (peace and blessings of Allaah be upon him).”
Imaam al-Nawawi said in his book al-Majmoo’:
“The prayer that is known as salaat al-raghaa’ib, which is twelve rak’ahs between Maghrib and ‘Ishaa’ on the night of the first Friday in Rajab, and the prayer of Laylat al-Nusf min Sha’baan, of one hundred rak’ahs – these two prayers are reprehensible bid’ahs. No one should be deceived by the fact that they are mentioned in the books Qoot al-Quloob and Ihyaa’ ‘Uloom al-Deen, or by the hadeeth which is mentioned in these two books. All of that is false. Nor should they be deceived by the fact that some of the imaams were confused about this matter and wrote a few pages stating that these prayers are mustahabb, for they were mistaken in that.”
Shaykh al-Imaam Abu Muhammad ‘Abd al-Rahmaan ibn Ismaa’eel al-Maqdisi wrote a very valuable book proving that these (reports) are false, and he did a very good job. The scholars spoke at length about this matter, and if we were to quote all that we have read of what they have said about this matter, it would take far too long. Perhaps what we have already mentioned is sufficient to convince the seeker of truth.
end quote
The original host [islamqa] of the fatwa is not displaying it properly so here is an alternative host to read it in full
[source]
Shaykh Fawzaan states:
"Nothing firm and reliable has been established on the authority of the Prophet, peace and blessings be upon him, that he held night-vigil prayer and fasted during the day of the 15th of Sha`ban. So, the night of the 15th of Sha`ban is like any other night, and if someone regularly observes acts of worship during other nights, then he may hold night-vigil prayer on this night without assuming anything special (because of it being the night of the 15th of Sha`ban).
This is because specifying a time for any act of worship requires an authentic proof, so if there is no authentic proof then the act is regarded as an innovation and all innovations are misguidance. Likewise, regarding specifically fasting during the 15th day of Sha`ban, then no (authentic) proof has been established on the authority of the Prophet, peace and blessings be upon him, to indicate the legality of fasting on that particular day."
Shaykh ibn Uthaymeen states:
"What is correct is that fasting on the 15th of Sha`ban or specifying it with reciting (the Qur'an) or making (particular) supplications on that day has no basis. So that day is like any other 15th day of other months. Something that is clear is that it has been made permissible for a person to fast on the 13th, 14th and 15th of every month. However, Sha`ban is somehow special (except for Ramadan) in the sense that Prophet, peace and blessings be upon him, used to fast more in Sha`ban than any other month. So he used to either fast all of Sha`ban or just a little.
Therefore, as long as it does not involve any hardship, one can increase his fasting days during Sha`ban, emulating the example of the Prophet, peace and blessings be upon him."