I would like to present to you the detail on fasting in Shaban / Shaʿbān and virtue of fasting it as much as possible:
عن عائشة أم المؤمنين رضي الله عنها قالت: ما رأيت رسول الله صلى الله عليه وسلم استكمل صيام شهر إلا رمضان، وما رأيته أكثر صياما منه في شعبان [صحيح البخاري (١٩٦٩) وصحيح مسلم (١١٥٦)]
ʿĀʾishah (may Allāh be pleased with her) said:
“I never saw the Messenger of Allāh ﷺ fast a complete month except Ramaḍān, and I never saw him fast more in any month than in Shaʿbān.” [Ṣaḥīḥ al-Bukhārī (1969) and Ṣaḥīḥ Muslim (1156e)]
عن أم سلمة رضي الله عنها قالت: ما رأيت النبي صلى الله عليه وسلم يصوم شهرين متتابعين إلا شعبان ورمضان [سنن الترمذي (٧٣٦) وسنده صحيح]
Umm Salamah (may Allāh be pleased with her) said:
“I did not see the Prophet ﷺ fasting two consecutive months except for Shaʿbān and Ramaḍān.” [Sunan al-Tirmidhī (736); Isnād Ṣaḥīḥ]
This indicates the virtue of fasting the whole month. The reconciliation between the report of “all” and “most” is that the majority of the month is often referred to as “the whole” in the language of the Arabs.
قال ابن المبارك (ت ١٨١هـ): جائز في كلام العرب إذا صام أكثر الشهر أن يقول صام الشهر كله، ويقال قام فلان الليلة أجمع ولعله قد تعشى واشتغل ببعض أمره [جامع الترمذي (٧٣٧)]
ʿAbdullāh bin al-Mubārak (may Allāh have mercy on him, d. 181H) said:
“It is permissible in the speech of the Arabs, if one fasts most of the month, to say: ‘He fasted the whole month.’ It is also said: ‘So-and-so stood the whole night in prayer,’ while perhaps he was eating dinner or busy with some of his affairs (for part of it).” [Jāmiʿ al-Tirmidhī (737)]
قال ابن حجر (ت ٨٥٢هـ): واستبعده الطيبي قال: لأن الكل تأكيد لإرادة الشمول ودفع التجوز، فتفسيره بالبعض مناف له، قال: فيحمل على أنه كان يصوم شعبان كله تارة، ويصوم معظمه أخرى؛ لئلا يتوهم أنه واجب كله كرمضان، وقيل: المراد بقولها كله أنه كان يصوم من أوله تارة، ومن آخره أخرى، ومن أثنائه طورا، فلا يخلي شيئا منه من صيام ولا يخص بعضه بصيام دون بعض [شرح المشكاة للطيبي (٥/١٦٠٤)]……وكان صيامه ﷺ في شعبان تطوعا أكثر من صيامه فيما سواه [فتح الباري (٤/٢١٤)]
Ibn Ḥajar al-ʿAsqalānī (may Allāh have mercy on him, d. 852H) said:
“Al-Ṭībī considered this (interpreting ‘all’ as ‘most’) unlikely, saying: ‘Because the word “all” (al-kull) is an emphasis to indicate inclusiveness and to negate metaphorical interpretation, so interpreting it as “some” contradicts it.’ He said: ‘Thus, it is to be interpreted as him fasting all of Shaʿbān at times, and fasting most of it at other times, so that it would not be mistakenly thought that (fasting) all of it is obligatory like Ramaḍān.’ And it was said: The meaning of her saying ‘all of it’ is that he would fast from its beginning at times, from its end at times, and from its middle at other times, such that he did not leave any part of it without fasting, nor did he specify one part for fasting over another.” [Sharḥ al-Mishkāt by Al-Ṭībī (5/1604)]…..“His ﷺ fasting in Shaʿbān was a voluntary act (performed) more than his fasting in any other (month).” [Fatḥ al-Bārī (4/214)]
Our Shaykh Muṣṭafā Amanpūrī (may Allāh preserve him) said: “It is recommended to fast in the month of Shaʿbān as mentioned in the ahādīth. If a person wishes to fast the entire month of Shaʿbān, it is permissible and an act of reward. This is supported by the practice of the Prophet ﷺ and the Salaf, and there is consensus (Ijmāʿ) on this matter.” [al-Iqtināʿ bimā Qāma ʿalayhi al-Ijmāʿ (p. 20)]
قال ابن الملقن (ت ٨٠٤هـ): قد قام الإجماع على جواز صومه كله، بل على استحبابه [الإعلام بفوائد عمدة الأحكام (٥/١٦٢)]
Ibn al-Mulaqqin (may Allāh have mercy on him, d. 804H) said:
“Consensus has been established upon the permissibility of fasting all of it (Shaʿbān), rather upon its recommendation.” [al-Iʿlām bi-Fawāʾid ʿUmdat al-Aḥkām (5/162)]
Regarding the wisdom of this practice, it is mentioned that it acts as a preparation for Ramaḍān.
قال ابن رجب (ت ٧٩٥هـ): وتكون منزلته من الصيام بمنزلة السنن الرواتب مع الفرائض [لطائف المعارف (ص ١٣٨)]
Ibn Rajab al-Ḥanbalī (may Allāh have mercy on him, d. 795H) said:
“Its status in relation to fasting is like the status of the Sunan Rawātib in relation to the obligatory prayers.” [Laṭāʾif al-Maʿārif (p. 138)]
See further the fatāwa of Shaykh ʿAbd al-ʿAzīz bin Bāz (may Allāh have mercy on him, d. 1420H) in Majmūʿ al-Fatāwā (15/ 416) and Shaykh Muḥammad bin Ṣāliḥ al-ʿUthaymīn (may Allāh have mercy on him, d. 1421H).
As for the Ḥadīth “When it is the middle of Shaʿbān, do not fast,” it has been weakened by many of the early Imāms as it contains ʿilal (hidden defects).
قال الإمام أحمد (ت ٢٤١هـ): هذا حديث منكر، يعني: حديث العلاء هذا [مسائل الإمام أحمد رواية أبي داود (٢٠٠٢)]
Imām Aḥmad bin Ḥanbal (may Allāh have mercy on him, d. 241H) said:
“This ḥadīth is munkar (rejected) – meaning this ḥadīth of al-ʿAlāʾ.” [Masāʾil al-Imām Aḥmad, Riwayah Abī Dāwūd (2002)]
وقال ابن حجر (ت ٨٥٢هـ): وقال جمهور العلماء: يجوز الصوم تطوعا بعد النصف من شعبان وضعفوا الحديث الوارد فيه [فتح الباري (١٢٩/٤)]
Ibn Ḥajar al-ʿAsqalānī (may Allāh have mercy on him, d. 852H) said:
“The majority of the scholars said: Voluntary fasting after the middle of Shaʿbān is permissible, and they weakened the ḥadīth narrated concerning it.” [Fatḥ al-Bārī (4/129)]
Our brother Abū al-Ḥasan al-Balūshī (may Allāh preserve him) wrote to me the Takhrij and Taḥqīq of this ḥadīth (prohibiting fasting after the middle of Shaʿbān) clarifying it’s weakness, in it he said:
والذي يظهر من خلال الروايات وهي تفسر بعضها بعضا أن المراد بالحديث ترك الصيام التطوع بعد النصف من شعبان كما في لفظ: ومن كان عليه صوم من رمضان، فليسرد الصوم ولا يقطع وهو أشبه بالظاهر. فعلى هذا، الحديث مخالف لما أصح منه فيكون ضعيفا خاصة إذا ضم إعلال العلماء على هذا الحديث والله أعلم
“What appears through the narrations – and they explain one another – is that the intent of the ḥadīth is the abandonment of voluntary fasting after the middle of Shaʿbān, as found in the wording: ‘…and whoever owes fasts from Ramaḍān, let him continue the fasts and not break them,’ and this is what is most apparent. Therefore, the ḥadīth contradicts what is more authentic than it, thus it is weak (ḍaʿīf), especially when the criticisms of the scholars regarding this ḥadīth are joined to it. And Allāh knows best.”
See the complete Takhrij here under the title (تخريج الحديث عن نهي ما بعد نصف شعبان).
Also I would like to present the benefit from his article regarding the bidah of Shab-e-Barāt, he (may Allāh preserve him) asked our Shaykh Ḥabīb al-Raḥmān Hazārawī (may Allāh preserve him) – who is from the students of Shaykh Zubayr ʿAlī Zaʾī (may Allāh have mercy on him, d. 2013) – while we were in this online group, regarding anything being proven from the Prophet ﷺ or from his Companions (may Allāh be pleased with them) regarding the individual status of the middle / 15th night of Shaʿbān. Our Shaykh (may Allāh preserve him) benefited us by saying:
لا يثبت في ذلك شيء بسند صحيح
“Nothing is proven regarding that with a Ṣaḥīḥ chain.”
So what appears is that it’s allowed rather recommended to be fasting in shaban as we have clarified.
The scholars affirm this and consider it a preparation for Ramaḍān. Weak reports regarding mid-Shaʿbān do not nullify its virtue as a whole.
As for fasting 15th of shaban, see the article [ There is no special virtue for 15 Sha’bān by al-Balūshī ] for more on that specifically.