Laylat al-Qadr (Arabic: لیلة القدر‎) (also known as شب قدر‬ Shab-e-Qadr , in Persian), variously rendered in English as the Night of Decree, Night of Power, Night of Value, Night of Destiny, or Night of Measures, is in Islamic belief the night when the first verses of the Quran were revealed to the Islamic prophet Muhammad.It is one of the nights of the last ten days of Ramadan. Muslims believe that on this night the blessings and mercy of God are abundant, sins are forgiven, supplications are accepted, and that the annual decree is revealed to the angels who also descend to earth. Shia Islam holds that Prophets and the Shia Imams are recipients of the angels on the night of Qadr and the decrees that they reveal.
Some commentators believe that Quran was revealed to Muhammad two times being 'the immediate revelation' happening on the Laylat al-Qadr and 'gradual revelation' across 23 years. The Quran uses the word Inzal (انزال) which justifies 'the immediate revelation', according to Allamah Tabatabai. However some others believe that the revelation of Quran occurred in two phases, with the first phase being the revelation in its entirety on Laylat al-Qadr to the angel Gabriel (Jibril in Arabic) in the lowest heaven, and then the subsequent verse-by-verse revelation to Muhammad by Gabriel. The revelation started in 610 CE at the Hira cave on Mount Nur in Mecca. The first Sura that was revealed was Sūrat al-ʿAlaq (in Arabic العلق). During the first revelation the first five verses of this Sura, or chapter, were revealed.

The specific date of Laylat al-Qadr is not mentioned in the Quran:

In Islamic countries and Sunni communities all over the world, Laylat al-Qadr is found to be on the last nights of Ramadan, mostly in on one of the odd nights (21st, 23rd, 25th, 27th or 29th) whereby night precedes day. Many traditions insist particularly on the night before the 27th of Ramadan.

Shia Muslims similarly believe that Laylat al-Qadr is to be found in the last ten odd nights of Ramadan but mostly on the 19th, 21st or 23rd of Ramadan with 23rd being the most important night. The 19th, according to the Shia belief coincides with the night Ali was attacked in the Mihrab while worshipping in the Great Mosque of Kufa, and died on the 21st of Ramadan. Shia Muslims regard these three nights as greatly rewarding. Many Shia Muslims, who make up the largest minority of Islamic followers — including the Ismailis and Dawoodi Bohras observe Laylat al-Qadr on the 23rd night of Ramadan, in keeping with traditions received through Ali and his wife Fatimah, Muhammad's daughter and the Fatimid Imams The tradition is also said to have been articulated by Ja'far al-Sadiq and other Shia Imams.