Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Responding to your question, Sheikh Kifah Mustapha, the Imam and Director of The Prayer Center of Orland Park and representative for Dar El Fatwa of Lebanon in the US, states:
As to the night of Al-Israa’ and Al-Mi`raj, there is no final authentic and clear Hadith that specifies it as it was related from Ibn Taymiyyah. Most of the narrations are in the level of weak (da`if).
What we know for sure is that it did happen as mentioned in Surat Al-Israa’ and it did happen before the Prophet (peace and blessings be upon him) made the migration to Madinah. Some scholars even believe that it might have happened in Rabi` Al-Awwal, based on some weak narrations.
As to the night of Sha`ban 15, there are few Hadiths about its virtues, but there are no authentic narrations of initiating special acts of worship during that night.
As for its virtues, the Prophet (peace and blessings be upon him) said, “Allah looks at His servants at the night of mid-Sha`ban and He forgives the believers and prolongs (matters) for non-believers and leaves those with grudges until they get rid of it.” (Ibn Hibban and Ahmad)
Regarding Laylat al-Qadr, we do have authentic Hadiths that it falls In the odd nights of the last ten nights of Ramadan like the Hadith of Abu Salamah reported in Al-Bukhari. Also the Hadith of Ibn `Umar (reported in Al-Bukhari too) which says that it falls in the last seven nights of Ramadan.
There are also Hadiths that mention specific nights, especially Ramadan 27, during which Muslims should observe Laylat al-Qadr. The Hadiths about Ramadan 27 were narrated by Mu`awiyah in At-Tabarani and declared authentic by Al-Albani and another Hadith by Ibn `Abbas in Musnad Ahmad and declared authentic with a sound isnad (chain of transmission) by Al-Arna`ut on the conditions of Al-Bukhari. What is meant by “find” is to seek it in order to do extra acts of worship such as Prayer, dhikr (remembrance of Allah), du`aa’(supplication), reading the Qur’an, giving in charity, etc.
Allah Almighty knows best.