Praise be to Allah.
Does Allah Come Down to the First Heaven on the 15th Night of Sha’ban?
This is mentioned in some ahadith, but there is some scholarly difference of opinion as to the soundness of those ahadith . There is no sahih hadith concerning the virtue of the night of the fifteenth of Sha'ban .
It was narrated from Abu Musa al-Ash’ari that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Allah looks down on the night of the fifteenth of Sha'ban and forgives all his creation except a mushrik or one who harbours hatred against the Muslims.” (Narrated by Ibn Majah, 1390)
The “one who harbours hatred against the Muslims” means one who has enmity towards a Muslim brother.
In al-Zawaid it says: (Its isnad is da’if, because ‘Abd-Allah ibn Lahi’ah is da’if (weak) and al-Walid ibn Muslim is mudallis.)
There is also some idtirab (weakness) in the hadith, as stated by al-Daraqutni in al-‘Ilal, 6/50, 51. He said: This hadith is not proven.
It was narrated from Mu'adh ibn Jabal, ‘Aishah, Abu Hurayrah, Abu Tha’labah al-Khushani and others, but the isnads are not free of some weakness, and some of them are very weak.
Mid-Sha’ban: Any Virtues?
Ibn Rajab al-Hanbali said:
Concerning the virtue of the night of the fifteenth of Sha'ban there are numerous ahadith, concerning which the scholars differed, but most of them classed them as da’if, and Ibn Hibban classed some of them as sahih. (Lata’if al-Ma'arif, 261)
Does Allah’s descending to the first heaven happen only on the 15th night of Sha’ban?
Allah’s descending to the first heaven does not only happen on the night of the fifteenth of Sha'ban, rather it is proven in al-Sahihayn and elsewhere that Allah descends to the first heaven every night, in the last third of the night. The night of the fifteenth of Sha'ban is included in this general meaning.
Hence, when ‘Abd-Allah ibn al-Mubarak was asked about the descent of Allah on the night of the fifteenth of Sha'ban, he said to the one who asked him: “O weak one! The night of the fifteenth?! He descends every night!” (Narrated by Abu ‘Uthman al-Sabuni in I’tiqad Ahl al-Sunnah, no. 92)
Al-‘Aqili (may Allah have mercy on him) said:
“With regard to Allah’s descending on the night of the fifteenth of Sha'ban there are ahadith which are weak, but the reports that He descends every night are proven and sahih, so the night of the fifteenth of Sha'ban is included in that, in sha Allah.” (Al-Du’afa’, 3/29)
Please read this detailed article by Shaykh Ibn Baz (may Allah have mercy on him) about the ruling on celebrating the night of the fifteenth of Sha'ban,
And Allah knows best.